Not allowed to e-file

When I select e-File in TurboTax Deluxe, the screen response is that my return contains certain forms that can't be e-Filed usually because of tax regulations.  But I'm trying to e-File the exact same forms that I've e-Filed for the past 3 years.  TurboTax does not tell me what these forms are.  I've researched on the IRS website and there is no form from my return listed as not allowed to be e-Filed.  How can I find out what the problem is and resolve it?