Its usual to see the nitrites rise slower than the ammonia. Once the filter is processing the ammonia it will convert it to nitrite which you will also need to do daily water changes to keep at a low level for the fish, which will eventually convert to nitrates (as Im sure you know). Keep an eye on the levels and preferably keep them below 0.25 - 0.50 for the sake of the fish. With a fish in cycle the whole process is longer than with a fishless cycle, but you will get there eventually.
You sound like you are doing a good job, keep it up and with time your filter will finish cycling then after a couple of week with ammonia and nitrites at 0 you can add a couple more fish.