If the lamps are old, that could certenly have an impact. Really though you should also be looking at other things like Phosphate and Nitrate.
Just so you know, the only algae that is affected by a UV is "green water" AKA: Phytoplankton. Those lamps only have a useful life of 6 months (ideally, but you could get 9 months to a year at *absolute* most).
Chris