Nitrites aren't showing up?

[MermaidGurl]

I added pure ammonia to my tank to start the cycling process on May 6th, and it measured 4.0 ppm.
It's now May 19th and ammonia has dropped to 1.0 ppm but nitrites still aren't showing up, what's going on?

 

[Delapool]

Sometimes the nitrites can come and go very quickly.