After you file

My Ex, claimed both our children on their taxes.  Our decree says that we each get one.  When I went to file, it rejected my efile.  I clear have documentation saying that each of us get one.  We have joint custody of our children.  What do I do now?!  My ex clearly told be they do not care what it says that they are going to claim both and since my ex did their taxes first now it rejected my e file....