I would add an ammonia source to your tank again. Get your ammonia up to around 4ppm and see how quickly it turns into nitrite, and then nitrate. Some folks like using raw shrimp as an ammonia source, but i prefer the liquid stuff.
Ace® Ammonia - 6 Pack - Industrial Cleaners - Ace Hardware
Ace brand ammonia works very well. Couldn't find the single bottle online, but it's a few bucks in store for 1 bottle. Go very easy on the stuff if you do decide to use it. I think a few teaspoons might be all you need.