As far as I know a cover can trap in CCs. But I am not sure HOW much they can trap, 18 sounds like a lot. But if you are holding chlorine and CYA, it's unlikely to be ammonia. I would leave your cover open for a day and retest? FC at 1.5 with a SWG at 27% also doesn't sound right and is too low a level with 60 CYA so something could be going on in the water to create CCs that are being trapped under the cover. I would assume you need to preform a SLAM
Also, it's hard for us to give advice with pool store results, as they are often completely wrong. Do you have your own drop test kit? It would be extremely beneficial to you to stop going to the pool store and instead order a test kit. Either the
TF Pro (uncheck the salt strips and ph meter) or the
Taylor k2006c. I would highly suggest getting the speed stir, too.
Both are expensive, yes, but they last a long time and in the long run, they save you time and money as you do not add the wrong things to your pool at the advice of the pool store with bad testing - which can results in a merry-go-round of chemical soup that ruins your water.
Also, fill out your signature with your pool info - see other posters for examples - this also helps us to quickly make suggestions to you.