Depending on what kind of test you're using, the Ammo Lock can cause a false reading. If the ammonia reagent that came in the kit says "Nessler reagent" on the bottle, you will get a false reading. With Ammo Lock, unless I am mistaken, the ammonia is still present...it is just bonded to some other molecule and rendered inert. Also, if your ammonia is very high, a 25% water change may not be enough to make a big difference. If you had a concentration of 4ppm, and you changed 25% of the water, you'd still have a 3ppm concentration. Don't forget also...you still have a nitrite spike to contend with after the ammonia is under control. This will take longer as the bacteria that processes nitrite grows at about 1/2 the rate of the bacteria that controls the ammonia. I think, and you'll need to verify this with someone else, that you can get a correct ammonia reading by using a salicylate type test kit...not sure I spelled that right, but it's close. Perhaps the LFS could test it for you and save you a few $.
Logan J