I tried recently to teach the jump shot to a beginner. It was surprisingly difficult for him, because his mechanics aren't developed enough to make a short and sharp stroke with the cue. He just couldn't produce enough force on the cueball with elevation to make the cueball jump enough to clear a ball. And yes, he had a quite good jump cue in his hands.
I got the impression that you already have a jump cue but just cannot get the cueball to jump, true ? If that's the case, you need to work on your mechanics. First, I'd say to practise with your playing cue first, but don't go into jumping yet. You need to work your mechanics and work on the basic beginner mistake: shooting (with elevation) and not to keep your body still. So instead of trying to jump, place a cueball a couple of inches away from the cushion and put the object ball at least 3' away from the cueball with a straight line into the pocket and shot being perpendicular to the cushion so you need to shoot over it. Don't put the object ball too close to the pocket to make things easier. Then, just shoot the object ball in with about 45 degree elevation and draw the cueball back a couple of feet with a medium or medium hard stroke and avoid jumping at any cost ! When you can make this shot with accuracy, your body should be stable and still enough to switch to a jump cue and correcting your aim a little bit higher to perform the jump, just add an obstructing ball at least 1' away from the cueball. You should have developed your mechanics already with the shot above and shooting with your bridge hand on the rail will make things a little bit easier.
Hope this helps.