Could be just as simple as where you park your car, under a tree in Autumn, maybe you live near the sea with a lot of sand in the air ?? basically if something can block your drains it appears that it will do. Having your car garaged and only using it in summer will give you a lot better protection from blocked drains, but if your car is left out in all weathers it seems more likely that you will end up at Durham Dave's or somewhere else the work can be carried out. I had mine done by Dave and it was in quite a state. I hadn't noticed any with its operation, just seemed like a sensible preventable measure