I can't see an obvious answer, but it's not clear to me how your lightbox works so may I ask some questions about it?
1) From the picture, it looks to me as if you're projecting the light upwards through glass. Is this correct? This seems like an odd design decision to me because if your negative is in a contact printing frame then you're introducing an extra (and unnecessary) glass layer.
2) This may be a dumb question, but are you sure those are UV tubes? I ask because every UV tube I've seen in recent years has been black not white.
Also, have you tried printing with another lightbox to get a comparison time? I'm half way across the country (Worcester), but you're welcome to use mine if the logistics work. PM me if you're interested.