Hi there, I sooooooo feel for you, I was right where you are twice now.... waiting and it looks like absolutely nothing is happening.....but please be patient. I will suddenly start changing and you will be on your way. The patients is well worth it, I had one tank that I was not patient enough with and eventually I just had to start from scratch as I constantly had to tinker with it and do daily water changes - it just never settled - whereas the 2 I cycled properly are a joy - stable 0 Ammonia and 0 Nitrite all the time.... but it took a good 6 weeks or so - I found turning the temp up a little helped -
What level of ammonia do you have in the tank? What are current pH and kH? What temp is it at now? Have you ever had to top up ammonia since the first dose? What is the current nitrate reading in the tank?
OK - I think I know what's happening. Right back at the beginning you used some Safestart if I remember correctly. We don't recommend this because the ammonia>nitrite bacteria that are correct for aquarium filtration have no dormant stage and thus cannot survive in a bottle so that makes it pretty pointless. However, there are also nitrite>nitrate munchers in these potions who can stay dormant for some time and will reawaken when temp is high enough, thy are in flowing water and nitrite is present.
What I think has happened is that the first stage (ammonia>nitrite) has begun to happen, but at a very slow rate because you only recently turned on the heater. Evidence that this stage has started is there in that the ammonia level is dropping. The small quantity of nitrite being produced at this early stage has re-awakened the dormant nitrite munchers who are present in sufficient numbers to process it straight through to nitrate before you are able to detect it on a test.
You can confirm this by testing your tap water nitrate level. If it is lower than the current level in your tank then my theory is likely to be correct as if ammonia falls and nitrate rises, there has to have been nitrite present in between, even if you didn't see it on test.