Almost all pool problems boil down to two issues. Not enough sanitizer in the pool or poor/bad filtering.
Do you have a sand or cartridge filter? If you have a sand filter you could try adding a little DE.
Backwash your filter, get it running and check the pressure, mix up some DE in pool water (1/3 cup of DE in ~gallon of water) and pour into skimmer. Wait 10 minutes or so and check the pressure again. You want about 1lb rise in pressure... repeat the DE add step until you see 1lb rise.
You could also ask the experts here...http://www.troublefreepool.com/
Mr1precision is spot on... keep it simple and DO NOT spend any more $$$ at the pool store. Although... the if you don't have a good test kit the first you should do is purchase one (one that tests for free chlorine, combined chlorine, total chlorine, ph, alkalinity, stabilizer)... Read everything on the above site and you'll be on your way to pool enlightenment.
Most folks there use the BBB method (Bleach, Baking Powder, Borax) and rarely make a trip to the pool store. I used the BBB method with great success until I installed a Salt Water Chlorine generator last year. Now Ijust add Borax and Baking Powder as necessary.