Why is my tank not cycling?

I am currently cycling a 20 gallon tank, fish-less by using fish food. I've bumped the ammonia levels up to around 3-4 ppm, but it has been over a month now and I still have no nitrites and the ammonia levels are not dropping. I don't know if it matters but I do have two live plants that have been in there the whole time, I even used some filter media from an already cycled tank in an effort to speed it up, any ideas on why I'm not seeing any beneficial bacteria yet?