You're using too much filling. That's a common mistake. Are you using anything else besides cheese? make sure all your ingredients are "dry" before using. Moisture encourages leakage.
and, as Amy said, refrigerate the whole thing to firm it up before cooking.
i got them ready-made from a local grocery store, so if they're overstuffed i guess i've just gt to deal with it. it's good to know that for when i make them myself though.