Manpower is recovered as a percentage to how much manpower you are missing. Your used manpower and your available manpower, cannot go above your total manpower when combined. So if you recruit more troops than you can support, they will stop reinforcing, and your manpower pool will stop refilling as well. So check on those values to see if you have a proper manpower growth or not (hoover over your manpower value to see all the information).
If it doesn't make sense, attach a save game and I can look at it.
If it doesn't make sense, attach a save game and I can look at it.