We have a intex 1500 pump/sandfilter.. it only works in neutral neutral position. We've changed the sand this year and that didn't help. The pump is 1 season old. We recently changed to filter balls.. still no improvement. Our pool is only a 10x16x3.5... the filter has been disassembled and cleaned twice this year and new O rings installed. Does anyone have any idea what the problem would be? Thank you...