Of the nine Apollo missions that went to the moon, 24 astronauts either orbited or landed on the moon. Only three of those astronauts went on 2 separate missions - the other 21 only went once.
Why was this the case? Once an astronaut was trained and proved himself in an Apollo mission, wouldn't it have made more sense to use the same astronaut in future missions, rather than taking a gamble at somebody who hasn't quite proved himself?
First of all there were 49 astronauts in the corps. The needed to do training for their missions months/years in advance for prime and backup crews. In the end though, it really depends on the astronaut who flew the early Apollo missions.
Wally Schirra (Apollo 7) was one of the Mercury 7 astronauts, he flew on Mercury, Gemini and Apollo. He was planning to retire after the flight anyway, but he caught a cold and his illness soured the three man crew's mood and they nearly mutinied during the flight, especially over a TV broadcast. The late Chris Kraft (who was flight director) swore the three would never fly again.
Don Eisele (Apollo 7) was having an affair before his flight and the program didn't want the embarrassment of a divorce (also, see Schirra above)
Walt Cunningham (Apollo 7) got kicked upstairs to head the Skylab program (again, see Schirra)
Frank Borman (Apollo 8) had no interest in being the first man on the moon and he felt that once Apollo 11 was successful the rest was icing on the cake.
Rusty Schweickart (Apollo 9) got sick on his flight (Borman did too on 8) and NASA hadn't pinned it down on Space Adaptive Syndrome, yet or understanding it. Not necessarily wanting someone who would get sick on the way to the moon, he became a guinea pig for motion sickness tests. He did fly in Skylab though.
Tom Stafford (Apollo 10) got promoted to take Alan Shepard's job as Chief of the Astronaut Office. Shepard had gotten experimental surgery on his ear, and was returned to flight status. Stafford also flew with Deke Slayton as part of the Apollo-Soyuz Test Project.
Michael Collins (Apollo 11) was offered back up commander on 14, and then commander of 17, but Collins turned him down as training and spaceflight put undue stress on his family.
After 11, crews (and back up crews) had already been assigned to future missions, three of which would be unfortunately cancelled by Congress before they could fly (which would have had 3 Apollo program veterans). In fact, Curt Michel got bumped from Apollo 17 for Harrison Schmidt who was a geologist, and the only geologist to walk on the moon.
The only one I'm not sure of is Bill Anders from Apollo 8.
Main Sources : Chaikin, Andrew (1994). "A Man on the Moon." Slayton, Donald; Cassutt, Michael (June 15, 1995). "Deke! U.S. Manned Space From Mercury To the Shuttle
This is my first answer on AH, I hope this meets the standards of the sub. If not, I understand.