Can your daughter explain why. Is there something she doesn’t like at her dad’s? A new partner? Or it could be she is growing up a bit and wants to be with her mates more. Most kids settle on one home or the other as they get older. It’s not unusual.
Talk to her, try to understand what is behind it and then explain to your ex and suggest how to keep everyone happy.
my ds didn’t like going to his dad’s because he missed his Saturday swimming lesson. So we agreed that his dad could spend most Sundays at my house, which gives me a day off, son sees his dad, and still goes to swimming. I go out or keep out of the way.
.
.