I seem to be having this same problem with my 5 gallon. My ammonia had been going down as it should but when I added ammonia again on Thursday, nothing happened. It pretty much stayed around 1 to 2.
After finding this on Saturday, I tested for Nitrite and it was 0 while the ammonia was still about 1 to 2. I did a complete water change and then added ammonia again, raising back to 1. That was Sunday morning and it has not changed again, still hanging at 1. Nitrite is still at 0.
What do I need to do to get the ammonia to go down? Do I need to do anything other than a water change and add fish. All I plan to put in it is a betta and maybe a couple shrimp but I don't want to stress them out or kill them. There are 3 real plants in it now, 2 anarchis and a moneywort. All 3 are almost tall enough to be to the surface.