Why is my main data being charged instead of my bonus data?

So I noticed before that my bonus was being charged anytime I browse while the main data was static. However, for like a week now, the reverse has been the case. I’m currently on the N8,000 2 month plan and I have 17gb as main data (reduced from 30gb due to sharing data) and 22gb left as bonus. So why am I being charged from the main data and not the bonus one?