I would suggest one thing. From about 90-100 feet have game of catch throwing only your change up as hard as you can. Try to hit the exact same place on your partner's body every time. Don't mix it up. Focus on that spot and continue to try hitting it over and over. That's the best way I've found to get a feel for my change up. Once you have done that successfully, throw a 40 pitch bullpen from a mound with only your change up as hard as you can throw it. Don't be discouraged if you miss your spots--don't sacrifice intensity for accuracy.
As you say, they have to believe it's a fastball or they aren't biting on it. When you slow your arm down in an attempt to throw it for a strike, they see the difference early.
Also, try a different grip at some point in the future if your current grip isn't getting it done and doesn't come around for you.