You probably need more experience building and maintaining web apps and general Ruby programming. There are three excellent webcast series that will put rocket boots on your feet. There's Avdi Grimm's RubyTapas,[1] Gary Bernhardt's Destroy All Software,[2] and Ryan Bates' RailsCasts.[3] I consider them required educational materials for the working Ruby/Rails programmer.
Most of the work I do is in java and ruby web apps. I've seen a lot of Railscasts, about 1/4 of DAS, and none of Ruby Tapas. I try not to binge on DAS because I prefer watching them one at a time and fully applying them before moving on. I think my problem is how big the codebase that Rails is and where to start contributing. I see your point to more experience but I'm not sure that's the crux of the issue.
I totally binged on the DAS screencasts. Then when I had trouble implementing anything, I'd go re-watch the relevant vids.
The reason I mention experience is because Ruby itself has a lot of convention that you only get really fluent with through repeated exposure. Just now I went and browsed to a random Active Record source file and had no trouble figuring out what was going on just looking at the source.
A year ago I would have been lost like you. But now I can take individual pieces of a big codebase and grok them without having to understand the rest of it. You only get that fluency through experience.
[1] http://www.rubytapas.com/
[2] https://www.destroyallsoftware.com/screencasts
[3] http://railscasts.com/