That's an interesting point. (Certainly there still is an attract there problem too, because I know a lot of atheists/non-believers/secular humanists/pantheists don't always know that a UU church can be a kind and gentle social option, just as the person I was responding to seemed to be seeking.)

I know that I've long considered myself a UU "irregular", which makes me "part of the problem". I show up as I feel like it when the whim hits me or there's a lecture/speaker I want to hear that week or I generally just feel a need for some coffee with a group I know I can typically count on to socialize with. Without the implicit coercion of "traditional" services have behind them telling people they must show up every week, UU can be seen as having a bit of a disadvantage on that front in keeping/maintaining/growing their communities over time. (The little UU church that is my favorite in my hometown is not quite 150 itself yet, I don't think, but probably shares a lot of similar tales of growth and shrinking over a nearly as long of a lifespan.)

At that point, so far as my advice may be, for what little it might be worth, finding repeat visitors that become irregulars that become regulars are a marketing problem like just about any other social club or an MMO videogame. There probably are things to draw from some of how those cope (or don't) with very similar problems. Help people feel welcome, help them feel engaged, help them find reasons to keep coming back.