Why does my MO return keep getting rejected?

I've never had this problem before.  It keeps saying an error with"Box 18" on W2.  I have gone through it several times, even deleted and started over.  The "Check my return" doesn't come back with an error.  And when it DOES reject and I go into to "fix my return" it just keeps going through a loop of "click here to fix" and back to "ready to file?", it never takes me to whatever the actual error is, so I can't fix what I don't know is wrong.