What is reach?

Reach is capped by your targeting. Narrow an audience enough and you'll exhaust it — everyone in it has seen the ad, and further budget just shows it to the same people again.

This is the constraint behind tightly targeted campaigns. A one-mile radius around a single venue might contain only a few thousand reachable people, and no amount of budget creates more of them.

When reach plateaus while impressions keep climbing, the campaign has run out of new people. That's the moment to widen targeting or refresh creative, not to raise the budget.