a coupel questions come to mind.
first, are you on city water, and if so does your water utility use chloramines?
second, what are you using for a water conditioner? some conditioners will release ammonium when they break chloramines, which will show up on an ammonia test.
third, have you tested your tap water? you may posssibly be changing out ammonia-rich water with ammonia-rich water (I had a similar problem a few months ago with nitrates)
fourth, do you have any fish in there? if so you need to do more frequent water changes to get the level down around or below 0.25ppm, as more than that is toxic to your fish. it may not kill thme right away, but it will definitely make them more susceptible to illness and weaker overall in the future. I would do daily 50% changes until that number goes down to 0.25 if you have fish in the tank.