Let's back the truck up.
It is likely that you have algae, even if you can't see it.
OCLT is overnight chlorine loss test. UV consumes FC. At night your loss should be minimal. If the loss is greater than 1FC (margin of error for the test), it is likely you have algae.
When you have algae (confirmed by the OCLT), you need to run the SLAM process to kill the algae. Link-->
SLAM Process
Your SWG is on. The SWG needs to be OFF in advance of performing the OCLT, and your FC need to be >= 3FC. The OCLT evening test needs to be AFTER sunset ad morning test needs to be BEFORE sunrise.