Getting people to use their vacation package

People already bought a vacation package, my job is to make them use it, they call to schedule it, However sometimes no one calls, so I start contacting them, sometimes they never use their package.
How do I get them to use their package or start having vacation plans?
How do I get them to call me back or reply to my emails?
How do I pressure them to secure their dates even thoug they are not sure about it?