Too many features open - Are all the valves adjusted correctly? Too much equipment on at one time will stop many cleaners from working (waterfalls, spa jets, surface returns, etc.).
Pump too small - The pump suction cannot overcome the total dynamic head (line resistance) in your pool circulation system.
Underground leak - If you have an underground leak, you can dig up some of the lines yourself. However, for generally less than $100 to $200 you can have a professional come in with special detection equipment that will pinpoint your leak including areas under the pool that you can't get to. Costs to repair the leak will vary according to where it is.