New player not appearing in game

A player just joined a game I'm hosting. I'm using pregenerated characters, when this player chose which character they wanted to control I went to assign it to them and they don't appear on the list of players in-game. But when I go back out to the game details page they are listed as a player of the game. I've reloaded the page a few times and even waited a while to see if it would work, but it didn't. In that time another player joined the group and I'm able to assign a character to them normally.  I really have no idea what's wrong. Does anyone have any suggestions how I could fix this?

Did the player in question actually enter the gameboard (not just join the game)? If they do not enter the game to where the map/journals/etc is at, they will not be fully registered in your game's database.

I've been on roll20 for years, so it seems like something like that should have come up by now. But I'll see if that works.

I've been on roll20 since the beginning and 9 out of 10 times I've seen this is, it is because the player did not go into the game itself but just joined the game and maybe posted in the game's forums. When this happens, the "in game" database still has not registered the player as a viable selection for the drop down menu selection.