I'm not sure what you've tried already, but your best bet would be to give them a VM just making sure that they actually want to continue in the league. If they do you should tell them what you expect of them, i.e. at least one post in the thread after each of their matches, or something alone those lines. If they say they want to be apart of the league, but still continue to be inactive even after you've "warned" them, then just kick them out, and post their names here so others can avoid having future problems with those mangers. If they say they no longer want to be apart of the league straight away, then just manage the team yourself until the end of the season, and find some new managers to replace them between seasons (if you're planning to continue on with the league). Since you're already over halfway through the season, I wouldn't bother trying to find any new mangers now, and instead just deduct the now manageress teams some points to ensure that they don't make the finals series. I know how frustrated you'd be because managers are even more crucial to a five team league, compered to eight to ten team leagues where people can probably get away with being not very active.
Even though it might be slight downgrading not having a great deal of activity in your thread, just make sure that you're enjoying running the league, because at the end of the day that's all that should matter.