You should not lose more than 4 ppm FC or so during the day. Maybe a bit more if the FC is well above target range for your CYA. But 17.5 ppm? That is a bit excessive.
I would suggest you do the OCLT again. Be sure to brush the pool, etc after you have added the chlorine for the night. Then test 30 minutes later.
I would suggest you do the OCLT again. Be sure to brush the pool, etc after you have added the chlorine for the night. Then test 30 minutes later.