As Hoseman suggested, try different grips. Tom Seaver used at least two different grips on his curveball. Interestingly, Seaver also taught to use the middle finger as the "power" finger in creating the spin, yet he himself used the index finger (so again, everyone is different).
You may also want to try a grip similar to the four-seam fastball, where you have the middle (or index) finger riding along one seam (to pull) and then lining up your thumb against another seam to add some "push".
Sometimes I have my students practice throwing a curveball using NO seams, then trying with seams. By not using any seams, you force yourself to use correct arm/wrist action to create the spin. Another thing I do is have the pitchers practice their curve with a short distance, one-knee drill using a softball. Again, to get the correct spin with the larger ball, you MUST use good mechanics.
And as someone else mentioned taping two balls together, you can try that, or, practice by throwing a taped-up, empty tennis ball can.