Just from my own experience, the cycling process can take a long time... especially if its a fishy cycle and you have to do water changes. My 20g long has just finished cycling... it took alittle over a month because my ammonia and nitrite spikes got so high I had to do daily water changes to keep them at tolerable levels. Although the water changes were keeping the fish safe, it was greatly slowing down the cycling process. Well now, ammonia is 0, nitrites 0, and nitrates 5; I'm so glad I'm through it though. I think you just have to keep testing that water and be patient. It's probably a combination of you changing the filter and the water changes (although yours or only once a week).