What are your water temperatures? Most of the SWG's have a cold water cutoff and stop producing chlorine when the water temp is below 58F. It could be that your water is warming up slightly during the day and your cell is only able to generate chlorine when the water warms. Most people that live with "year-round" pools switch to manual chlorination in the winter months.
However, because it is also cold, your chlorine loss rate should be very minimal. I only lose maybe 0.1-0.2ppm FC per day. If your FC is at target levels and then drops to 0ppm in less than a day, then you might also have an algae problem. You can test that by doing an overnight chlorine loss test (OCLT) -
Pool School - Perform the Overnight FC Loss Test (OCLT)