So,
I have discovered that when players are inactive in a game that I GM, just sitting around waiting for me to do something for them to react to, I hold a large part of the responsibility for my stagnant game. Possibly most of the responsibility. This isn't about Ars Magica, or problem players, but about GMing.
As a GM, I need to know what the players want and don't want. If I deliver what they don't want instead of what they do want, they will either wait for me to do something interesting (from their point of view) or drift away from the game.
As a GM, I need to empower players, so that they feel comfortable enough with the world to take initiative. If a player thinks his initiative will be stomped on, or continually corrected, or limited to rubber-stamping the GM's intentions, he won't bother. He'll wait for something he finds worthwhile.
As a GM, I need to work with the players to make sure they create characters who make sense for the campaign I intend to run, who belong in the world, who have something they want to do IC, who have the means to start doing it and, above all, interest the players. To the extent I fall short of this in any way, I will get lackadaisical players of lackluster characters.
As a GM, I need to remember not to bog the players down in details when they take the initiative or flex their power. This one dooms many campaigns to a lingering death.
I'm not saying you do any of these things... I'm talking about me, not about you.
If you think there is a possibility that any of the above might also apply to you, you might want to talk to the players, not about their roleplaying, but what they are hoping for from you.
Some thoughts: Only let a PC run a Quaesitor if you are willing to let his opinion of Hermetic Law largely represent the truth in your game. When a player prefers to have his character hang out in his lab, it means that the game world is less interesting than his lab;. A SG probably needs to to something to be sure that characters have a balance of lab versus adventure time that works with your campaign and that is consistent with the other characters. If a player designs a magus who spends all his time in the lab, he'll need another character to do something during adventures, but if his magus spends a lot more time in the lab and library compared to other PC magi, that character will be a lot more powerful than those others. Finally, most rpgs are played at a level where interest is generated by discovering whether a PC can do something, but AM can be played so that the question is more about how the PC does it, or what he decides to do.
Anyway,
Ken