We had our pool company change out the sand in our filter, 2 days later we opened our pool ourselves for the first time. It went well everything seemed to be working. When starting to vacuum winter debris from bottom it just wasn't vacuuming well. It was a little dirtier than usual but my vacuum usually is pretty efficient. I usually don't have to backwash but maybe twice on the opening vacuuming. This time I did it 5 times. Did notice that my psi is running at 20 -25 while in the filter position yet in backwash or rinse it runs 12-15 psi. I'm thinking something is wrong with the sand filter. The service guy says he expected to use 3 - 50 lb bags of sand but put in 4 he also said he only took. He only took out what was dirty as he thought he may not enough sand. Any thoughts?