Perhaps it is because his subjects -- playful little girls, ladies of high fashion, picnics on the beach -- seem a little bit too sweet. Perhaps it is because his soft and subtle colors go so dull in reproductions. Maybe it is only because the spirit of his work feels less American than French. Whatever the reasons, posterity has been less than fully fair to the painter Maurice Prendergast (1859-1924). It is true he has his champions, some of them devoted, but his small and gentle pictures, particularly his monotypes, have never quite received the wide fame they deserve.

Thanks to Ambassador Daniel J. Terra, that's about to change.

Terra, 73, is President Reagan's ambassador-at-large for cultural affairs. He also is the founder of the Terra Museum of American Art, which is housed, temporarily, in Evanston, Ill.

Terra has been buying. He's bought two sites across the street from each other on Michigan Avenue in Chicago, on which he plans to build a museum (he plans to link the lots with a bridge built over the street). He bought Samuel F.B. Morse's 1832 "The Gallery of the Louvre" (it cost him $3.25 million in 1982). Yesterday he said, "I just bought 13 Whistler etchings, yesterday, in New York." Terra has been buying the monotypes of Maurice Prendergast as well.

He owned one in 1981. In 1982, he bought 36. He bought nine more in 1983, and another nine last year. He now owns 55. They are quiet little pictures of unexpected loveliness. All will go on view on Sunday in a touring exhibition that was previewed here last night at the National Gallery of Art.

"Monotypes of Maurice Prendergast From the Terra Museum of American Art" is a touching, oddly eerie little sweetheart of a show.

The Prendergast we meet here is something of an alchemist. In 1891, in Paris, working with the common, the entirely familiar, he began to make something wholly new.

His sources are apparent. Such single standing figures as "Lady With Umbrella," "Lady With Handkerchief," "Lady in Red," "Red Haired Lady With Hat," their uninflected backgrounds and the monographs beside them, clearly owe a debt to the woodblock prints of Japan.

He has borrowed, too, from Whistler. One sees that in his street scenes, his misty skies and seascapes. "Esplanade," the first picture in the show, and the first monotype he made, is a night view of a figure seen in moonlight, beside water. It might well be called a nocturne. Prendergast also was inspired by Bonnard and Vuillard and the other Gauguin-inspired Post-Impressionist Frenchmen who had studied, as had he, at the Acade'mie Julian.

His loose, suggestive brushwork, with its conscious imprecision, is, in spirit, French. Prendergast was not one of those Americans who liked sharp-focused crispness. Like other Boston painters who turned to modern art in the last half of the 19th century, he found himself attracted to the brownish vaguenesses of Barbizon School painting. He loved to spend his weekends sketching out of doors. "In those days," said his artist brother, Charles, "Maurice was hell on cows!"

He was just as good at dresses. In 1873, at 14, Prendergast left school and took a job wrapping packages in a dry-goods shop. "Whenever he could spare a moment from the paper and string," wrote Van Wyck Brooks, "he sketched the women's dresses that stood about the shop. Nothing amused his eye more than a pretty dress, blue, green, yellow, or old rose, as one saw in all his pictures to the end of his life."

If one tries to imagine a blend of Japonisme, Whistlersesque mistiness, Gibson-girl high style, Post-Impressionist color and Barbizon School vagueness, one calls to mind a mess. But somehow Maurice Prendergast made that mixture sing.

Two factors served Prendergast as catalysts. One was the edge-blurring, color-dimming, aerating power of the monotype technique. The other was his almost spooky, almost symbolist turn of mind.

Monotypes, though prints, are also partly paintings. Asked how he made his, Prendergast offered these directions: "Paint on copper in oils, wiping parts to be white. When picture suits you, place on it Japanese paper and either press in a press or rub with a spoon till it pleases you. Sometimes the second or third impression is the best." The method, which calls particular attention to the brushwork of the painter, allows him enormous freedom. He can draw in the paint, or wipe it away. At almost every step, one has the right to change one's mind.

The pictures that result are far more colorful than etchings, yet more intimate than paintings done on white paper or cloth. From a distance, Prendergast's monotypes look ghostly. In the act of printing, all their once-bright colors have somehow become veiled or subdued.

These pictures, although charming, are almost never easy. One has to peer into them to read them. There are passages in some that are entirely illegible. It is their strange mood one remembers most, not their technique.

Cecily Langdale, who wrote the catalogue, speaks of Prendergast as if his art were always jolly. In his "domain," she writes, "life is idyllic and every day a holiday; it is a world of circuses and boat races and festivals, one inhabited by frolicking children and captivating, fashionable women."

But those captivating women tend to walk alone. They often turn their back or look blankly at the viewer. One can't quite read their faces. These playing children, too, play in isolation. A mood of symbolist alienation often undercuts the jollity. Prendergast's colors are somber and shadowy rather than sunny. His pictures have no laugh tracks. Instead they make one think of the distances and troubled souls in the art of Edvard Munch.

An exhibit of 55 monotypes by some lesser painter might soon bore the viewer. But this show never drones. Terra has done his young museum a great service in assembling this collection. (The National Gallery, in contrast, owns only one Prendergast, a 1898 watercolor, a recent gift from the artist's sister-in-law.) Terra said yesterday that his show will visit 17 museums -- "60 more have asked for it" -- after closing here April 14.