Don't just ask: do.
Employ guerilla research techniques:
Grab yourself a clipboard and go find any one of your users - Sit next to them and say do you "I'm just going to watch how you user the software so I can find ways to improve it" and take notes.
Take the opportunity to raise the subject of the software whenever you get a chance to chat to a user - It doesn't matter what the original chat is about: "So, how are you finding the software?", and listen to what they have to say.
Attend training sessions for new users to find out how they adapt to the software.
If you ask for test candidates very few people will volunteer unless there's something in it for them - 5 minutes chat for a coffee and a cookie, 15 minutes for a free lunch... it's expensive so you'll need a budget if you want to go down that route.
It's far easier just to go to them rather than asking them to come to you.
If you want to test prototypes then do exactly the same - go to users and tell them someone is trying to improve the software and want to know their opinion on what's been done so far (always tell them it's someone else's work so they're more honest with you).