Fitbit question--running vs walking

The past couple of days I've done running in place (Wii Fit) and all my steps aren't registering.  Today I kept checking my fitbit and sometimes it didn't change even while I took eight or ten steps, and then would start up again, but not registering them all.

Are you supposed to change something when you run instead of walk?

I love my fitbit!  Maybe your fitbit is skipping steps because, there are two ways to run in place... one where the chest is bouncing (for lack of another word) up and down, the second is when the legs move, but the chest stays pretty stationary.  For the first, the fitbit will register the movement, the second one may not jar the fitbit enough to advance the steps.  This has been my experience.  

The legs are moving but the chest, not so much.  The Wii was picking up every step--said I ran over 3.5 miles. (I ran for 20 minutes.)  The fitbit logged about 2500 steps, if that.