I have had this issue before and after asking about the frequency with which I would see ads I was told that the setting was the upper limit.
I have been keeping track and I have now gone 15-20 hours of browser activity without seeing an ad. If this is not a technical problem of some type, I would suggest that asking to be shown ads should be explained in a way that does not mislead Brave users.
The logic I potentially like about Brave is not blocking ads and cookies. There are multiple ways to do this. I am interested in a service that balances the protection of my privacy with the ethical commitment I feel to providing revenue to content and service providers. I have been waiting for Brave to reach the point at which this is possible if I turn over my ad revenue to the sites I visit. I have never come close to achieving this goal despite being a very heavy browser user.