Getting internal participants

I'm working on a internal system for a company at the moment and I'm having huge trouble recruiting participants for user testing.

That this is a system they themselves will have to use when it goes live does not help much in recruiting people, with only a fraction of those invited ever agreeing to attend. The trouble is people are just too busy with the work they have to do now to worry about their work next year.

How can I turn things around? How can I convince these internal participants to agree to attend my user testing sessions?