Something is removing the ammonia. It isn't processing naturally or you would have nitrite in increasing amounts. I'm not certain what may be the problem since your dechlorinator does not have an affect on ammonia. I did see where a product Aquasonic make called CHLORMON does eliminate ammonia instantly. You've got me stumped on this one. I don't think continuing to add ammonia will have much effect since you aren't developing bacteria to process the nitrite. On the other hand, if your tank is processing all the ammonia and your nitrite isn't ever rising, then maybe you would be ok. One other note, are you using liquid test kits or strips? Sometimes, strips are very inaccurate, especially if they are old.