Why do you believe this to be the expected behavior? Your block rate is primarily dependent on the client using the Pi-hole and your internet habits. 30% is completely reasonable, and is likely to change hourly.
As a single point of comparison (and to illustrate that the block rate isn't why you are seeing ads), I have four Pi-holes running now. Each serves a different subset of clients, and I don't see ads Block rates as follows for the past 24 hours:
28.7%, 39.8%, 56.9%, 16.1%.
Where are you seeing ads? Some ads (i.e. YouTube) cannot be blocked by Pi-hole since they are served from the same domains as the content.