Is this a monster tank? I'd say the life in your tank just wasn't enough to create a "real" cycle. A bit of an ammonia spike as you stated wouldn't really be enough to create much of a spike in nitrates either.
Chuck in a couple of cocktail shrimp. 3 chromis aren't going to do much to cycle a bunch of dead rock and some "live" sand. You need them to either poop more or add the poop yourself (in the way of the cocktail shrimp
).
Just my two cents
