The same issue happened to me when I attempted to print MD state returns -- it turns out a couple of the forms I am using will only be E-File ready this Wednesday.  I don't know why there are no errors to flag it before I go to print.

I did print out PA returns for a client over the weekend, so I think your return may have one unique form that is not yet E-file ready.  Go through each form in the return and you should be able to spot it.