Another way, is to check the FC level after the sun goes down and then check it again before the sun comes up. Of course this requires the SWG to be running all night. If you get a big increase in chlorine level overnight, it's working. If no increase then run the same overnight test again, with the SWG off. If the overnight FC loss is less than 1 ppm, then you can be pretty sure the SWG is not working. Obviously, it the overnight FC loss is greater than 1 ppm, you've got something growing in your pool.
What FC level do you normally try to maintain?
Are you using your automation to tell you what your FC level is? It could be a bad ORP probe. What are you using to manually test your chemicals? How about providing your current reading:
FC
CC
pH
TA
CH
CYA
Thanks,
Jim R.