The only thing that could make the test inaccurate would be CO2 in the atmosphere reacting with the sodium hydroxide solution that you use to titrate your sample. The other ingredient, phenylphthalein, the color indicator, should last parctically for ever. If you keep the cap tight on your sodium hydroxide bottle, it, too, should be good for years. If you have the cap loose or off for long periods of time, CO2 in the atmosphere could neutralize much of the NaOH, by reacting with it forming sodium bicarbonate (NaHCO3) in just a few weeks. Try taking one 20 ml sample and letting it stand in the room for 24 hours. Then titrate that and compare your results with those from a freshly drawn sample. Most of the CO2 should diffuse out of the 24 hour sample to the room air. When it is in equlibrium with the room air, it should be close to the end point and have, at most, a few ppm CO2. The freshly drawn sample, on the other hand, should have a lot more CO2. If your 24 hour sample reads 35 ppm, then either you have a lot of weird acids in your tank water or your NaOH titrating solution is shot. It shouldn't be too expensive to have LaMotte send you a new bottle of NaOH solution.
Regular 50% weekly water changes would make the organic matter insignificant. the mattern it self is not the issue, the tannins and humic acids in high amounts might, but it would take a fair amount and few to no water changes to accomplish a large effect.
The pH/KH method is better suited as far as a method for CO2.
There are other reasons possible for a lack of pearling.