First thing I would check is rollers and tubes (dirty, broken, etc) next I would check the calibration of the pumps using a scale also sample water with a scale; next I would check with reference solution 3 tests back to back to back. All should be within range of 7.5 +-4 ( it think). As long as you get 3 tests the same you should of solved issue. At that point you can adjust the difference with +-. Hope this helps. You should also check if calibration is accurate do a dosing of 20ml of each pump using a scale it should read 20ml if not double check hoses, etc.