nvidia - Bumblebee's optirun command was working fine, then suddenly it does nothing anymore

I installed Bumblebee today on my Dell laptop (i3 2370 + GF630M) to play Doom, using Doomsday. I tested the "optirun" command with firefox, then with doomsday and all was working fine. Now however, when I type "optirun doomsday" or "optirun firefox" in the terminal, literally nothing happens. No error message, the process just hangs for eternity, unless I terminate it. Any ideas?