Sounds pretty normal to me.
12 days is not that long when it comes to a fish tank. How big is the aquarium, what do you have in it to cause the ammonia to rise, and how high is the ammonia at the moment. Changing 10 % of the water will only lower the ammonia by about 12 % which is will be hard to measure. Moreover, whatever is causing the ammonia to rise will probably continue to keep the ammonia rising.
Also, depending on your local water supply, there may be Chloramine in it. This will also give you a high ammonia reading.
I would continue with the partial water changes making sure not to disturb the tank unduly until it's cycled. If you have something very ammonia tolerant in the tank (you haven't mentioned whether it is saltwater or freshwater) such as Damsels or Danios, I'd be tempted to just leave the tank to cycle itself.
More info please! Jeff