Short Answer: Check that the
batteries are properly installed, the battery compartment insert (if one
exists) is in place, and the compartment cover is securely closed.
Long Answer: Some
battery-powered vibrators come with a foam, rubber, or cardboard insert
that fits in the battery compartment to keep the batteries in place and
prevent them from rattling. If the insert was cardboard, you may have
accidentally discarded it while unpacking the vibrator or changing the
batteries. If necessary, you may be able to improvise a new one by
cutting a piece of scrap cardboard to fit inside the compartment.