I don't know what kind of humidity levels you have in your printing environment, but it could be a factor-
the same neg that takes me 6.5 minutes in the summer with full humidity can take 20-40 mins in the coldest driest months of winter. If that is not the issue, I'd go with Bill's suggestion of changing your bulbs. Home Depot sells blacklight fixtures and appropriate replacement bulbs that work just fine, and are quite cheap.