Did you just recently open your pool? If so, did you find that the CYA had dropped over the winter? If so, then what may have happened is described technically
here (and also
here) under "CYA Degradation by Bacteria" where soil bacteria get into a pool that has no chlorine and they multiply and consume the CYA. For every 10 ppm CYA, they produce 3 ppm ammonia and this takes 24-30 ppm FC to get rid of it. In practice, some of the ammonia dissipates (either outgasses or gets taken up by algae). When you add chlorine to a pool that has ammonia, at first it raises the CC and the FC fairly rapidly goes to zero -- it seems like an insatiable chlorine demand. At some point, the FC starts to measure but there is still CC which may be where you are at now. Then, the CC drops and the FC stays stable when you add more.
If you find this situation when you open a pool, you can get an ammonia test kit at a pet/fish/aquarium store and then use 8-10 ppm FC for every 1 ppm of measured ammonia.
The 10x rule that Taylor and others talk about to get rid of CC is technically incorrect. It is based on needing about 10 times the ammonia ppm reading. CC is measuring monochloramine (mostly) and in different units (ppm chlorine gas, not ppm nitrogen). The actual amount of FC needed to get rid of CC is only a little more than half of the CC level. Using more chlorine makes it go faster, but in your situation, you probably still had ammonia in the pool that would form more CC as you added more chlorine.
At this point, I'd just keep adding more chlorine until it FC holds at some shock level, at least 10 ppm or so. If you want to get an ammonia test kit, you can do that, but you might be at the point where all the ammonia is now used up and has formed CC.
Richard