I have an in ground 14 x 28 pool that is about 15 years old. I have been maintaining it myself and am running a salt generator. All of a sudden, I find that the water is not being pulled into the skimmer. I checked for blockage and there is none that I can see. I have back washed twice and rinsed.. I am showing about 14 # of pressure which is normal. The pump reservoir is full of water and looks normal as well. What am I missing? Thanks in advance.