A.J.
I had a similar problem with my now 5 year old a few years ago. My daughter felt embarressed to tell her father that she had to go to the bathroom when he had her that she was having accidents all the time. Maybe your daughter doesn't feel comfortable telling other people that she has to go. You never know what may be going through her head "are they going to tell me no?" "what if I can't make it" etc. It is something to think about. Maybe have a talk with her and find out if she is having problems talking to the sitters.
I hope this is helpful.
~AJ