It means you're tipping your pitches.
According to the NPA, batters can judge up/down, left/right direction of the ball after just the first 5 or 6 feet of its trajectory. On the other hand, it takes quite a bit longer to judge the speed of a pitch. However, if batters figure out that when they see your pitch elevate that you're throwing an offspeed pitch, then you've lost the advantage that it takes longer to judge speed. In other words, batters can see your pitch elevate and know right away that you're throwing an offspeed pitch allowing them to adjust their timing sooner than they otherwise would.
It's hard to say what the fix is without a side view but I'll take a stab and suggest that you should focus on throwing your offspeed pitches using the same tempo and arm speed as your fastball. Everything - mechanics, timing, arm slot, arm speed, release point - should be the same asyour fastball. But many pitchers slow down when they throw offspeed pitches which shortens the stride and raises the release point (which, by the way, is not a good thing when you're throwing a curve). You may have to adjust your grip or pronate more to take more off of your change-up. However, your curve will probably break sharper and later and that's a good thing.
