Work through these on My Roster:

1. Gender. A gymnast entered as Male will never appear in a female level, and the other way round. Open the roster entry and check it. MeetMaker now names the gymnast and the mismatch on the registration page.
2. Level. The gymnast needs a level recorded for the same organization that sanctions the meet. A gymnast with only an AAU level will not appear for a USA Gymnastics meet.
3. Organization number. Some events require it before an athlete can be entered.
4. Already registered. Someone already entered in this event does not appear again.
5. The level is not on the meet. If the level itself is missing from the event, only the event director can add it - do not change your roster.
6. Imported from AAU as a coach. AAU reports adult athletes as non-athlete memberships, and on an AAU import those rows default to Coach. Anyone imported as a coach is not an athlete and will never appear on a registration page, whatever their gender or level says. See Importing an AAU roster directly.

Fix the roster, then return to the registration page and reload it.