I have two daughters I share. One just turned 16 and the other 17. The 16 for the past 2 years doesn't always like going to dad's. He only gets them 1 day every other weekend. She loves him. She just doesn't wanna go. It causes her stress. It's just too much going on for her. She likes doing her homework or working on her art. If she doesn't go for a month, then she will go. Cause she misses him. Or she will call him up to ask to go. I have sol custody and have had it since they were 2 and 3 so it's little different. When I started letting them go for visit they were 10 and 11.
Don't force her. Talk to her about why and keep encouraging her to spend time with you all. She may also be opposed to summer vacation with her father as it might take her away from spending the summer with her friends/in the home she is comfortable in. I'm sorry you are going through this. I know this pain so deeply myself.