Ex won't stick to pickup schedule and it's messing with my kids

So my ex has been consistently late picking up the kids on my days off. Like 30 mins to an hour late every other week. I've tried talking to her about it but she just makes excuses about traffic or work running over. My kids are getting confused about when they're supposed to be ready and I'm stuck in limbo. The custody agreement clearly states 6pm pickups. Do I need to document this and bring it to my lawyer or am I being petty? I just want consistency for the kids sake.