This is where your problem is, not your fault just bad design of the filter.BB (beneficial bateria) is what changes the ammonia into nitrites and then the nitrites into nitrates. The nitrates are then removed through water changes.
With your filter, the best place for your BB to colonize is the fitler cartridge, which you change regularly. There for when you change your filter cartridge you are removing a large portion of your BB. This is the cause for your ammonia.
One way to fix this is go buy a package of 4 pot scrubbers, they need to be the nylon plastic type with no added chemicals. Stuff these into your filter prefferablely after the cartridge(this will give your BB a permanite place to colonize. When your change your cartridge only rinse the pot scrubbies with tank water(the chlorine in tap water will destroy the BB). Try this for a while and if it does not help I would go with a different brand of filter, either aquaclear or penguin bio wheels.
In the mean time to combat the ammonia use Prime(it will detoxify the ammonia)and keep doing you water changes.