Tips on getting invasions as a Darkmoon? Or is it just bugged?

In my experience, getting summoned as a Darkmoon or Blue Sentinel is damn near impossible. I can stand and wait for over a half hour and not get one invasion. Aldrich Faithfuls however, the invasions are pretty much back to back. Are the Darkmoon and Sentinels bugged, or are invasions really just that hard to get?