Something is not right here and maybe more info would help.
When did you install the UV light? Before or after your slam?
When did you start having problems with fc holding?
What led you to slam initially?
What were you using to sanitize the pool before your slam?
How could you have completed the slam if your FC did not hold overnight? (See pool school for the complete details of "shocking and maintain" (slam) which include FC not dropping overnight be more than 1)
From your reports of FC not holding perhaps the UV is contributing but perhaps other factors are exacerbating: eg
- that polyquat 60 in large amounts can cause an increase in chlorine demand
- if you were previously using something like baquacil system or hydrogen peroxide to sanitize or shock chlorine is not compatible and "conversion" is required
- if the pool condition prior to your first slam a few weeks ago was swampy and you have ammonia in your water from cya breakdown, it will take a HUGE amount of chlorine to break it down and make the water hold FC (been there, done that and you have to keep chlorinating without stop til it holds)
So while removing the UV entirely is a obvious experiment to try, the fact that you got the UV light in response to a problem suggests that the uv light may not really be the problem.
The more info we have on the circumstances and prior pool maintenance items used, the better the chance we can give meaningful info