I suffered from this problem for several seasons. CM tend to bit a lot more miss than hit in general. My current team has a crop of four young-ish CM (plus one auto generated one season oldie as back up) who are pretty reliable now. The way I managed to achieve this was by purging my entire MF one season and then signing four/five young CM. I kept the ones who were good, and packed off the duds pretty quickly and replaced them with other CM. This season one of them is showing seasons of being useless, while another one who I was intending to get rid of has come good again.

In short, go for maybe overloading on CMs then be ruthless and get rid of the ones who are no good, keeping the cream of the crop.