The thing is, there is a limit to how much you can earn from sponsored images in a certain time period. Maybe a particular picture pays only the first time it seen? Anyways, when i Refresh tabs on my pc, it cycles through 4-5 sponsored images and gives 0.01 bat each time.
After all have been seen once, it repeats the same pictures again. Ive noticed that it also stops paying BAT. If i try after an hour or so, it starts paying again.
This theory is based on personal experiences, so might be worth messaging someone from the brave team to answer your query with technical facts.