President Reagan's scheduled visit to Muncie, Ind., where he was to address a convocation of Ball State University students Wednesday, has been called off, and White House officials are giving conflicting reasons for the decision. The official reason is that there were "logistical difficulties" in setting up the visit, but one senior White House official said that the real obstacle was First Lady Nancy Reagan, who thought her husband needed more rest after his 13-day Pacific trip, which concluded last Wednesday.

Instead, Reagan plans to hit the road later this month, speaking to what an official called "a reasonably Middle America" audience of young people during the commencement season at a site yet to be determined. "We're trying to cement the positive feeling young people are demonstrating to the president and tentatively to the Republican Party," said White House political liaison Mitchell Daniels. Lobbying 101 . . .

One of the more curious animals among Washington newsletters is "TCI Lobbyist Letter," published by the Tyson Capitol Institute. Subscribers pay $150 to receive, 10 times a year, four typewritten pages that have featured such basic advice as, "The lobbyists most likely to succeed with senators and staff will be those who are flexible and who understand the politics of the stand they are asking lawmakers to take. Those lobbyists who take a position and hold it or who ignore political realities may find themselves frozen out, say Hill staffers."

In the May issue, the TCI Letter takes up the question of self-preservation. "Many corporate lobbyists," observes its lead item, "have two jobs: lobbying Congress and lobbying their home offices to justify the existence of the Washinton office."

The newsletter advises bringing executives to Washington for "field trips . . . for a first-hand look at the legislative process."

"The hook for a D.C. visit should be linked to a major shift in policy, economic direction or regulatory trends . . . ," says the letter, while counseling a second agenda, "to impress on the folks back home that Washington is here to stay." Public Announcement . . .

Ever since Linda Chavez resigned from the White House as director of the Office of Public Liaison, speculation has been rife about who would succeed her in one of the few White House jobs consistently awarded to a woman.

Yesterday, the president announced appointment of Mari Maseng, 32, a former Reagan speechwriter and onetime assistant secretary of transportation who has most recently worked as director of corporate relations for the Chicago-based Beatrice Companies. She also worked for the 1980 presidential campaign of Sen. Robert J. Dole (R-Kan.), and later for the Reagan-Bush campaign. Her title will be deputy assistant to the president.

Others who have served the Reagan administration in the post, which bears responsibility for liaison with women, minorities and religious groups, are Secretary of Transportation Elizabeth Hanford Dole and U.S. Ambassador to Switzerland Faith Ryan Whittlesey. Chavez resigned to run for the U.S. Senate in Maryland. Thou Shalt Not Close . . .

Last week the Labor Department announced which six of its Job Corps training centers it had chosen to close in compliance with new budget restrictions.

But at around the same time the announcement was being made, the House of Representatives added to its supplemental appropriations bill an amendment that would prevent Labor from carrying out the closings. The one-paragraph amendment, introduced by Rep. Michael L. Strang (R-Colo.), both prohibits the closings and also expressly prohibits Labor from exceeding Gramm-Rudman-Hollings funding levels to keep the centers alive.

Why? Because, according to Strang and Rep. Mike Synar (D-Okla.), who supported the amendment, Labor has $300 million in construction funds remaining in its fiscal 1986 budget, along with $12.3 million it doesn't plan to use for the pilot program to which it's allocated. The purpose of the amendment is to force Labor to spend those available funds on the Job Corps Centers.

Synar, who launched the challenge to the budget law's constitutionality now before the Supreme Court, said of Labor's decision to apply its budget cut to the jobs program, "This is an example of what happens when computers make decisions about people, and this is why Gramm-Rudman is wrong."

He also noted that one of the threatened centers was in his state.