The only problem is not lack of quality and merit worthy posts. Amidst low quality posts, there are still posts which cross the average benchmark that ought to receive merits. The problem could be that most of our merit sources have high standards for giving out merits and it's rare for their standards to be met by an average forum user.
Merit sources are human. They can't see and be all over the place ready to merit anything and everything that deserves it. Don't forget that it's a manual task, you can't automate it. That's why there will always be posts that remain unmerited somewhere, but I don't think high standards are the reasons for that. Plus, there is a thread in the Reputation board where anyone can report posts that deserve merits to merit sources. If you feel like you deserve more merits than you get at times or the same is true for someone else, feel free to use it.