Pool Filter Losing Pressure/Won't Stay Primed

Hello. We have an above ground pool with a Hayward cartridge filter and pump. When we prime the pump and turn it on...the water bubbles near the top. After about a 1/2 hr or so, it is no longer near the top and sounds like it isn't getting enough water. We have cleared the hoses, resealed the lid and the hose, made sure the propeller in the pump was clear. We just don't know what else to do. Could the problem be related to the filter? Could the air release valve be leaking air? Help!! Pool pump is only 1 year old. Filter is 7 years old.