I've contributed to hundreds of projects, and maintained a ton, so i feel this pain, so please take what i say in that vein :)
The question i usually ask when i see this is: "What is the thing you did that you think should have caused people's behavior to change?" Assuming you want this to be different, what actions are you taking that should have changed this? If they didn't, what did you learn and try?
I'm not suggesting you will get to some perfect place where all contributions are great, but if you really want to improve them overall, you have to be doing things to improve the behaviors you see, seeing whether they work, and iterating, until you get the behavior you want out of most contributors.