I would remove the floating tab dispenser. Add enough liquid chlorine to get the FC level up. Then I would run the OCLT test as suggested above to rule out algae. If the pool passes that test, I would run an OCGT (overnight chlorine gain test). You would run your cell at 100% and keep your pump running. Since you've ruled out algae with the OCLT, then the FC level should increase overnight. If it doesn't, then your cell is probably bad.