Hi ... new to site so please bear with me,

I have a small garden pond with a bio filter and external uv supplied by a more than adequate pump. The pond has been up and running for a good two years now and I have generally had no problems but recently the bio filter has started to frequently block up and stop water flow. I have unblocked it twice in the last two weeks which is unusual ... can anybody shed some light on this or post any suggestions as to what could be causing this. Cheers.