Attachment is normal not personal
No matter how much time daddy spent with the kids they preferred me. No matter what. It's normal for kids to have an attachment to one parent over the other. They loved their daddy but wanted me near. It phases. Still to this day there's nothing like mama for them.
I felt badly for their dad. Must feel awful to have your children cry when left with you every time. Even though its not personal it can fee pretty rotten.