The first thing I can think of is you are testing your water incorrectly or using old reagent. Are you using strips or the proper black-dot test?
Isocyanuric acid can only be added by using Tri-chlor tabs, Di-chlor tabs or powder, or powdered CYA. It cannot be increased by adding bleach.
So either your testing has a problem or one of those chems was added inadvertently.