Did you test the water coming directly form the cell? (Turn off the system and loosen a union on the cell to collect the water from the side of the cell going back to the pool when the cell is on and generating.) The chlorine should be very high when tested with OTO--dark yellow to orange to brown color. This will insure that the SWCG is working properly. If not you should try cleaning the cell with acid as per your manufacturer's instructions.

In any case, drop your pH t0 around 7.6, shock the pool, and I believe that you will find that your chlorine will hold after that. You probably have a nascent algae bloom that is consuming the chlorine as fast as it is being produced and shocking will destroy it and allow the SWCG to catch up and maintain the pool (assuming the system is properly working, hence the test suggested above). With a CYA of basically 100 ppm you want to adjust the cell to maintain the FC at NO LOWER THAN 5 ppm. 6 ppm would be a comfortable level to aim for. Also, do not try and test pH until the FC level drops below 10 ppm after shocking. HIgh chlorine levels will cause a false high reading on pH tests.