Why wouldn't a baby mouse walk?

they are to young to walk they should start to walk after a few weeks. Depending on it age it could not be walking simply because it is not old enough. Or it can be timid, the runt, or maybe something is wrong with it. Its hard to tell with no real details. To avoid these sorts of problems try getting mice from good breeders. They keep track of the mouse's heritage so they avoid sickly mice.