The UV bulb is still working but is it true that the power/strength of the actual UV gets weaker as it gets older, thus not killing the algae as it passes, keeping the water brown?
You've hit the nail on the head, I change my UV bulbs every year. Bit expensive but worth it. Also depends on the size of your pond and the size and type of your filter. Might be worth considering 'beefing' up your filter system.
Change the UV bulb every March. Then use the UV until the end of October then turn it off for the winter. Change the bulb again in march and switch it all back on.