Just a thought. As the child of seperated parents (they were never married). My mother and I lived maybe 30 minutes from my father.
When ever it was time to visit NEITHER wanted to do the drive. (I worked all day, I'm tired, I don't have the gas, I don't feel like it, I have other things to do... etc. etc. etc) They never arranged it a head of time. They mostly refuse to talk to each other. So there I was with the phone in one hand negotiating this for them. TWICE each visit! Once to GET to his house and again to get HOME.
IT SUCKED! I felt like CRAP! I felt like neither parent really wanted me.
So, just remember the kids stuck in between all this is all. Or, I'm sure you all have thought of that. So maybe a gentle reminder to the ex is what is needed.