Ex won't stick to pickup schedule

So my ex has been consistently 20-30 minutes late to pickups for the last month. She's not even apologizing about it anymore, just acts like it's normal. I had plans with my kids today and had to reschedule because she showed up at 4:15 instead of 3:45. How do I handle this without getting the court involved? We were doing okay on communication but this is getting frustrating.