Like any other progesterone-only pill that you take every day, it basically works by preventing ovulation.
Normally, if you ovulate and don't become pregnant within a few days, you will have a period about 14 days after ovulation as your body gets rid of the uterus lining it had built up just in case = you have a period.
Without any ovulation, all that is disrupted and bleeding can happen semi-randomly. Some women get long bleeds and some don't get any, and this can change over time..
.. in other words, this is normal.
If you're worried or don't like this, see your GP. Switching to another progesterone-only method like the implant or another pill will probably have an effect, but it's not possible to know in advance whether it will be better or worse. Switching to a combination pill should mean you get regular light bleeds.