Sounds like your doing a good job keeping the ammonia low and your fish safe. With our "modern" way of sterilizing our drinking water it seems like it takes longer to get a cycle then it used to. You could try one of the bacteria boosters to help or just continue as you are and it will come.
I’m not sure if I have my tank cycled or not because I am testing for nitrates and I am not sure if I have 5 or 0 the color is hard to read from the chart. My nitrite is .0 and my ammonia is .25 but I am pretty sure nitrates are 5 I put in bio spira about two weeks ago.
If you have healthy plants that are growing they will be using the ammonia nitrites & nitrates. Have you done any water changes and if so about how much. It could just be that the plants are using the little waste the Guppies are making and you won't get a reading