With President Carter's nomination in the bag, most of Virginia's delegates to the Democratic National Convention here took the afternoon off today to go shopping and sightseeing in the Big Apple -- most of them, that is, except Lt. Gov. Charles S. Robb of McLean.

Robb, considered Virginia Democrats' best hope to recapture the governor's mansion in 1981, spent the day and much of the week, it seems, preparing for not one but two speeches to the party's assembled delegates.

That meant frequent absences from delegation meetings and floor sessions. So, while Robb made two appearances before the entire convention tonight, they didn't stop some Virginia delegates from grumbling about Robb's failure to do the right kind of politicking in the right places.

"He'll take a weekend trip to go to Danville to cut a ribbon, but you won't see him much among the delegation," groused one Virginia convention-goer who asked not be identified. "He's an enigma that it totally captivating political discussion in the state. I'm tired of talking about it."

Robb, 41, began staking his claim to the party's gubernatorial nomination three years ago when he was the only Democrat to survive statewide election. Before that he had been known chiefly as the late President Lyndon B. Johnson's son-in-law.

His party connections now are impeccable. He moves easily among national and other state party leaders. When Carter delegates from Virginia and 12 other states were invited to the White House the week before the convention, Robb was called to the front of the room by Vice President Mondale to share in the attention and media exposure.

Some of that is going on here -- as tonight's convention appearances showed -- and there are those who say Robb's national visibility is an indication that the Carter forces don't want to lose Virginia this fall as they did in 1976. Virginia was the only Southern state to vote Republican.

"It's a two-way street," Fairfax County Supervisor Sandy Duckworth said today. "Chuck Robb as the son-in-law of Lyndon Johnson has something to offer the convention, the party and President Carter. In return, the Democratic Party has offered him an opportunity to address the convention and given him national exposure for his future campaign. That's good politics."

The convention speeches aside, however, several Democrats here wonder whether Robb really knows that.

"I spend 30 percent of my total political life talking about Chuck Robb," a delegate reported warmly. "His name comes up at every cocktail party, with every person, with every group, and I don't initiate it. Church groups, law partners, people in the state party -- they all have a universally perceived question about him."

The question, the delegate explained, is why doesn't Robb act more like a politician and a man who covets higher office.

The answer, the delegate said: "Nobody knows."

"I've been asked to get to Chuck, to talk to him about it," said one Northern Virginian here. "I've refused. The guy has chartered his course and you're going to see it through 1981."

"He's low key," says John Melnick, a Carter delegate from Arlington and a former member of the Virginia General Assembly. "There are two types of politicians -- the glad-handers and people like Chuck who are serious."

Robb clearly sees himself as a serious politician, a state official interested in making speeches only if they're "substantive." Even among his friends -- which is how one delegate characterized the Virginia gathering in New York -- you won't see Chuck Robb working the crowd.

In contrast, Robb's all-but-official GOP opponent for next year's governor's race, attorney General J. Marshall Coleman, likes nothing better than an old-fashioned, back-slapping, hand-pumping session with voters.

Instead of working the convention crowd, Robb says he spent most of the week meeting with State and Defense department officials who briefed him in preparation for his speeches. He also spent his mornings and evenings attending numerous receptions for prominent or up-and-coming Democrats like himself.

"They were all planned well ahead of time," Robb said of the various events that kept him away from Madison Square Garden and the morning meetings of his delegation, of which he is honorary chairman. There was a dinner at "21," a breakfast with an investment firm, an NBC luncheon and receptions hosted by Time and Newsweek magazines.

"I've met most of the governors and House members and Democratic senators who are here," Robb said. "Everybody has their own agenda. After my speeches, I expect to be able to be on the floor more."

Robb's speeches -- one opposing a proposed freeze on development of nuclear weapons and the other a spirited pep talk for the party -- were praised by most Virginia conventioneers, who erected a makeshift "Robb for Governor" sign in the delegation.

Privately, however, some of Robb's biggest boosters and some who admit they find him "a mystery" say his absences were noticed and not always excused.

"A whole lot of people are concerned because he's not been coming to the delegation meetings," said one prominent state Democrat. "I'm sure all of us have dinners and receptions and appearances, but we're here. He is our titular head, our leader. It's always very comforting to know your leader is here."

A Fairfax Democrat looks to Robb as "the caliber of candidate who can win in Virginia." But this same party official says Robb has been urged to "loosen up" and start acting more like a campaigner.

"When he goes out campaigning, he does a good job," adds Duckworth, a Virginia national committeewoman. "He's got charisma."

Melnick says Robb's wife, Lynda, has been on the floor often during the week and that he sees the couple "as part of us, not as celebrities."

Yet a reporter who tried to ask Lynda Robb her opinion of the recent platform fight over strengthening the Equal Rights Amendment could not get an answer. The woman who replaced Bella Abzug on the President's Advisory Commission on the Strains of Women gave only a terse, "No comment."