Cross Project Contributions

In my work with GEF I usually check out the newsgroups when looking for a solution to my current problem. I have noticed more than once that the solution exists in GMF, but wasn't contributed back to GEF and instead exists only for GMF users. I am really curious as to how this happens - especially when one of the stated goals of GMF is to contribute to GEF where possible. It seems that contributing fixes back to the source plugin would have several obvious benefits. Besides making these things available to a larger audience, this would make for more modular source trees and easier code maintenance.
I'm wondering if this exists in other projects as well and what can be done to help this problem (or if this isn't really a problem). I'm sure part of the issue has to do with the usual, time constraints and lack of resources, but here it would seem that it could actually be more efficient to back-contribute rather than trying to copy a chunk of code across projects and modify it to fix a bug or add a feature.
It is really not my intent to pick on specific projects - I only use GEF/GMF as an example because its one with which I'm familiar. And I really appreciate all the work that's been done there to date.