It may just not have dissolved, but it could be other things, as well:
  • How did you add it?
  • If you put it in via the skimmer, did you clean the filter within the 72 hours AFTER you added it?
  • How many hours per day is your pump on?
  • What -- mfg & EXACT product name -- did you add?
  • How did you test? ('Guess-strips' are bad at best, but when testing stabilizer, they are at their worst!)