Odds are that the chlorine levels are too high and are bleaching out your test strips. You don't say what method the pool store used to test, but if it's not drop-based, then that could be the issue there, too. Another possibility is that you have a huge chlorine demand from CYA that was converted over the winter to ammonia. When that happens (and it's happened to a lot of folks this year), it takes massive amounts of chlorine to overcome. What was your CYA last year, and what is it this year so far?

Have you tried using a cheap OTO kit from WalMart (uses red and yellow drops for pH and chlorine). Absolute worst case, you can dilute your pool water with distilled water and test that, and at least get some confirmation of whether or not there is chlorine in the water.