Normally this is caused when the item is not regularly cleaned or maintained. Back flush the unit and replace the sand.

How to back flush:

• Turn power off (important, should not run the pump dry, will cause damage to pump)
• Close Port A using cap (switch from Port D to Port A basically)
• Attach hose to Port D
• Put the control valve to backwash position (<< symbol)
• Turn pump on until water turns clear (or around 3-5 mins)
• Turn pump off once water turns clear
• Remove old sand and refill, or rinse existing sand (water droplet symbol – rinse for 1 min)
• Once complete, remove cap from Port A and apply to Port D