Usually cycles can take up to 6-8 weeks to fully stabilize and mature, your phase right here is completely normal. Every cycling experience is different, so you can only base it off your experience.
Just keep on adding ammonia to 4ppm once it's near the 1ppm mark. Just wait a while, and then your nitrites will show up sooner or later. I don't think evaporation can cause a huge nitrite downfall, either. Bacteria LOVE warm places, they also like a lot of O2, (oxygen) a filter on high, and dark places.
Good luck, a very experienced cyclist called Hallyx is here, that member can really solve your problem.
(Is cyclist even a word in terms of tank cycling? Lol)