My pool slide water is connected to the water returns in the pool. I have an on/offf ball valve to turn water on/off to the slide. Good pressure to the pool but water to the slide has very low pressure. I cut the hose to the slide and water is being pushed to the slide but not enough pressure to come to top of slide. I have back-washed pool several times with no change noticed. If you put your hand over one of the returns in the pool there is enough back pressure for it to work correctly. what would cause low pressure like this to the slide?