I think Donald's third suggestion is the most likely the key to your situation.

I don't think the problme is the glass. There are differences in UV transmission of glasss but unless the original glass of your contact printing frame was a speciality glass that allowed tranmsision of more UV radiation (not likely since these types of glasses are fairly expensive and not at all common).

Also, I also don't think the problem is a bulb mismatch, unless for some reasons you have a mismatch of normal output ballast with high output or very high output tubes. Again, this is unlikely since the difference in cost between normal versus high and very output tubes and ballast is so great that you would probably have picked up on the difference during purchase.