Lineup submissions must be completed and received by the server more than 15 minutes prior to the scheduled start of the game (since that's when the game actually starts simulating).
In addition, you cannot submit more than 2 positions with a blank starter or the coach will completely disregard your set lineup and build a lineup out of the players you have active for the game.
If neither of those occurred in this case then you may need to wait for a response from a BB.