Bathtub faucet leaking
In some cases there may be a gasket or o-ring that may have cracked or fallen out. In other cases replacing the whole spigot may be the only option, unless you can actually get a replacement part for just the diverter assembly that actually fits your particular spigot. 

It's often quite difficult to figure out exactly what model you have, which makes searching for parts rather difficult. The ones I have personally fixed, I replaced the whole spigot.