Also, try this: Get a small sample and add a drop or two of acid to the sample to see if the sample turns yellow especially where the drop of acid enters the water.
Ok I think I see our problem here, or at least part of it the Spinlab will only read FC up to 15 ppm WaterLink Spin Lab - 3576 - Pool and Spa - LaMotte so the pool store test is likely at the limit of there test and off the scale high. Your FC may be much higher than we thought, but before trying to add more hydrogen peroxide, lets do an experiment, take a sample (4 to 8 ounces) and add a few drops hydrogen peroxide to it, stir let sit for a few minutes then test it. You should see low chlorine levels if any.
Ike
ps
James W. Test is another good one to try.
p.s. I have been doing some reading online, and it seems that at pH levels below 7.0 the reaction rate of hydrogen peroxide and chlorine is greatly slowed. I have not found anything that defines how much below 7.0 this happens at or by how much.