document it. every time she changes or breaks than you write it down.
keep a log on it and save those phone messages. anything to prove your case. the phone messages might not be permissible but ya never can tell with some judges. also try to e-mail her and if she responds, print it out.
then go back and let the judge know..

Interference with visitation is a crime and she can get in a lot of trouble for doing it. You need to contact your attorney and let him know that your ex is not cooperating with the visitation. Keep details on what happened, when it happened and any other information that would be useful in getting the courts to get after her. Since it appears that this is something that happens a lot, she is in some serious trouble.