Ex won't stick to pickup schedule - exhausted

She's been anywhere from 15 minutes to an hour late picking up the kids almost every week. I have plans, I have a life outside of being a dad and I can't make any solid commitments anymore. Already tried being nice about it, already tried the text reminders. Do I need to go back to the mediator or am I overreacting? This is the one thing we agreed on in the custody agreement.