Does it happen only in the first game of the day, or any first inning?
Our 12u P1 had this issue off and on...until we realized that days we had a rec game in the morning (where she would usually get an inning or two in) followed by a travel game/games in the afternoon- problem disappeared.
On travel only days we had her throw a simulated inning or two beforehand.
Typically the first inning of the day. They have enough pitchers in TB that they'll split starting games on any given day. She has come in for relief from time-to-time and will do well.