In my experience, you cant EF if you have federal withholding but total income on the return is less than the withholding.

I have clients with excludable difficulty of care income that have withholding on it.  If I exclude all the income, we cant EF.  But if I only exclude enough income that leaves income of $1 more income that withholding, I can EF just fine and it doesnt make any difference to their tax situation.