S,
Most likely your filter is dirty..
When was it last cleaned?
Thanks,
Jim R.
S,
Did it work fine for a day or so after you cleaned it, and now it is no longer working?
What color is your water?
Did you add any floc or clarifier to your pool?
What is your filter pressure?
In almost all case, if things work after the filter is cleaned and then so after the flow decreases again, it is algae, even if you can't see it yet.
Thanks,
Jim R.