I have struggled with this same problem for a couple of months. I have a couple of tanks that have been battling ammonia levels. When I tested my tap water it shows around 1.0 ppm with an API drip kit for freshwater. I have tried Nutrafin, Amquel, Aqueon & Prime on it and it continues to read about 1.0 ppm. After spamming several boards about this issue and trying to understand why I was told that these products don't reduce the ammonia in the water, but rather neutralise it ot make it non-toxic to the fish. I have asked several people to see if they get the same results with ammonia in their water and what their readings are afterwards and so far no responses.
I did one test that makes me suspect of the whole thing. I took a bowl of tap water and began dosing it with Prime (supposedly one of the better ones). I dosed it with 0.5 cap fulls at a time. In the end it took two cap fulls (which should treat about 100g of water) to get the ammonia to read zero (from 1.0). While I can't justify (both as a risk to my fish and my wallet) hitting my water 5g changes with several cap fulls of prime at a time I just find it very suspect on what these products are supposed to do, and/or what I am dealing with in my city water.
Hope all the best for you. So far I have just been doing regular water changes (dailies) with hope that my nitrogen cycle will eventually be able to keep up with the water changes which dump 1.0 ppm ammonia in the tank.