The effective life span of an average uv bulb lasts approx 3 seasons (March through October) or 18 months. After that, it's time to replace- even if it's "lit".

Clean your quartz sleeve. Remove any calcium buildup, and if it's dry and still hazy, replace it. The quartz sleeve needs to be clean to transfer as much wavelength to the algae as possible.

Slow your water flow. Sometimes your water flow is too much for the strength of the uv unit you are using. By simply slowing the water flow, you will increase contact time, and be more effective.

Cool off your pond. Algae thrive in environments in excess of 58 degrees farenheit. Many hobbiests prefer a uv shade covering over their pond to deter predators, cool their water, and promote an environment that allows a social gathering spot even on the hotter days of the summer.