I'm going to assume that your personal endowments are capable of handling the ball properly - hand size, shoulder and arm strength, and so forth.
From my experience, one of the most common mistakes of sending a change-up that dies in the dirt, is the improper release point that the hand actually lets go of the ball.
Before I go any further, I'm not a advocate of the "release point" as a point, per say, with respect to the pitching cycle. However, there is a time when the hand lets go of the ball and the pitching cycle's influence on that ball.
So, remembering that the ball is going to have less "oomph" on it than would otherwise be the case, the pitch has to be released slightly higher and a bit earlier than the pitch it's trying to represent. Also, since the grip takes away a lot of the velocity on the ball, you'd better be ready to use a lot more body motion into the pitch to send it way. If not, your change-up will end up a fluff served on a platter.
Therefore, a slight elevation with the pitching hand, sending the pitch away, should correct your problem. By the way, yours is a common problem with pitchers who are learning the change-up for the first time, even in their mature years. In addition, don't forget to concentrate on targeting your pitch. Don't just send it down range with no specific place to go. If you do that, you're throwing - not pitching, on top of that your throw will be to an abstract "place", without purpose of location.