Welcome to TFP!!!
It may be that you need to backwash the filter.
It may be that you have algae, even if you can't see it.
How are you testing your water?
Can you post a full set of results from your water test kit? FC, CC, pH, TA, CH, CYA?
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!