A couple of thoughts.
Firstly, fish in tank cycling is much slower than fish less because you are forced to keep the ammonia levels low to protect your fish. In fish less cycling ammonia is added to give about 4 ppm
to encourage bacteria growth.
Secondly, the nitrite to nitrate bacteria a lot longer to establish than your ammonia to nitrite.
Lastly, are you water changing with conditioned water, chlorine and chemicals on tap water will kill the beneficial bacteria in your filter.
You could try adding a bacteria culture, there are many types on the market, but better still, do you have any fish keeping friends who could give you some 'dirty' filter material which you could put in your filter or tank to help seed your filter? Your cycling needs a kick start.
If you do lose your fish, and I hope you don't, then consider cycling the tank without fish and only add fish when your tank is ready.
Best of luck.