That is what I thought was possibly happening. However, do these sand filters need topping up from time to time - if so, roughly how often?
Generally you won
Generally you won't lose much sand over the years through backwashin, maybe a few pounds or so, but if you want to try something like Filter Aid, which is a fine powder that you add to the filter, it coats the top layer of sand so that it will catch smaller particles like you're experiencing. It sometimes works very well and is always worth a try if you don't want to pump that much water out of the pool. Adding more sand really won't solve anything as it will still pass through that sand as well and then back in to the pool through the returns.
That's a great help - much appreciated.