A few things may be at work here. First, if water is replaced in the pool as in an overflow situation, the CYA will drop which could reduce the chlorine generation of the cell. Second, high PH will also reduce the chlorine generated by the cell. Third, scale on the cell will also reduce generation. You did not post CH so I can't tell if this is a problem or not.

I would first inspect the cell for any scale and clean it too. Next, I would drop the PH and see if that doesn't fix your problem. If you still are low in chlorine, then you may need to increase the CYA.

Also, you can get a separate CYA drop kit for a more accurate measurement.