Why do I not receive ads consistently?
There is no guarantee you will receive ads with any regularity.
Ad campaigns may start at any time, but they also finish too. New campaigns may also target different platforms to the campaigns that were previously available. Each ad can also have a maximum number per day you can view, and they have a maximum number of times you can view them in their lifetime.
For regions with many active campaigns this should be a little less of a problem because there will be more ads which Brave can potentially show to you, but for regions with not so many active campaigns it means you will often receive a day with more ads, followed by one or more days with less ads.
You can view the current state of ads in your region, broken down by platform at https://www.kenherbert.dev/brave-ad-stats/.