How far are you trying to move it? Vegas has a feature called “snapping” that assists you in lining events up with other elements — the cursor, markers, timeline divisions, other events, etc — by instantly gravitating to such things when you’re really close to them. If the position you’re trying to move the event to is close enough to the event’s original position (which was already snapped to something), this may be why you’re having trouble.
To remedy this, either a) temporarily turn off snapping by going to Options > Enable Snapping to uncheck it; or b) zoom in your timeline using your scrollwheel or by clicking the plus sign on the bottom right of the timeline.
Rob