Player no longer appears in "Can Be Controlled By"

I've run a few sessions with the same game room and players, but for some reason this time that we are making new characters, one player doesnt show up in the "Can be controlled by" list. They also noted that the character sheets seem to be glitching a bit in so far as they seem to close randomly or don't always allow them to edit. They've made character sheets in this game in the past, and I've not had issues with assigning them a sheet before. Any thoughts? 

First thought: Check their name in the game, as opposed to the user name they start with. If they have changed one, you might be looking for the other. Second thought: Check that their name is not problematical: look for control characters like @#$%, etc. Look for hidden spaces at the end, or double spaces. Finally, make sure they haven't duplicated a name already in the game. I.e. in D&D, don't name yourself or your character "Goblin".

My rule of thumb when dealing with roll20 in general is to always make a new session every month and move everything into the new session, their system does not handle well when you have tons of bloat in the journal and everyones got a cache from loading various pages in this specific session and nobody clears their cache.  

Conversely, I've been running the same game for 3-4 years and not had to rebuild. YMMV. I'd check likely candidates before going nuclear. :D

Thanks for the quick responses! I went to check and I heard from my player that they had left and rejoined the game, and now their name is loading in normally. So we may never know what caused it. lol Still seems to be having issues on their end with the character sheet itself though, but the errors dont seem to be consistent so I'm going to work with them a bit more to troubleshoot the problem.