Hmmm, if it's just the flush can you do the, 'if it's yellow, let it mellow' thing? Potty would work too and you can empty and flush later.

She obviously knows a wee is coming, at least some of the time...can you remind her of the rewards again? Up the excitement about what's in the box? Take her every hour (this worked with DS as he got Jack of being interrupted every hour and then told us in no uncertain terms that he would tell us when he needed to go, lol!)?

Have you tried reusable pull ups - they look more like undies but are absorbent like nappies so you won't have to clean up accidents all the time?