It sounds as though you still have some organics (algae) consuming your free chlorine. One way to TEST this is to measure your overnight loss of chlorine.

I would suggest that you dose your pool to "best guess chlorine for CYA" "shock" level around 8 or 9 pm. Then check you CL around 11 pm or so (give it time to distribute) and then re-check in the am. If there is nothing bad (algea or others) in your pool then your nightime CL and morning CL should be the same (this is a negative test or a normal)

If there is a drop, that means that something undesireable is consuming your free CL (this is a positive or abnormal test).

I would suggest superchlorinating until your morning CL does not drop.

For this test (overnight chlorine loss test), you will need to turn your SWG off. Because if you leave it on, you may get a false negative test. I would run it the rest of the time.

I have no experience with SWG, but it seems that your CYA is too low. Most manufactures suggest a CYA in the 80 range.

I cannot explain your loss of CYA, but must consider a measurement error. How did you measure your CYA? Most kits cannot give a reading less than 30.

I hope this helps.