I have a pretty good curve, almost 12-6. My change is good when its on. Somedays its just not there. I have been playing around with a slider for a while. I throw mostly over the top. with the slider and sometimes my 2 seam i move the slot out to the three quarter. My slider drops about 4 inches straight down, no side movement. What am i doing wrong?

Jim Brosnan once described the slider as a breaking pitch that is not as fast as a fastball nor as sharp-breaking as a curve but that is easier to throw and to control than either of the other two. It’s really a “feel” pitch, and my guess is that you don’t quite have the feel for it yet. So let me tell you a couple of things about how to throw it.
First off, there’s the grip. It’s something like a two-seamer but very much off-center, with the index and middle fingers very close together and the middle finger just touching one seam, the thumb uncerneath for support and on the seam. You throw it like a curve, but roll the wrist, don’t snap it—just turn it over, kind of like a chef flipping a pancake or a crepe—easy does it. When I learned to throw that pitch, I got the hang of it in about ten minutes but I realized I wasn’t going to “get it” overnight, so I worked on it for several months and finally got comfortable enough with it to try it in a game. And one thing that really helped me was that I was a natural sidearmer—an exasperating and infuriating one who threw everything that way, and I never had any arm or shoulder problems.
So my advice is just keep working at it, and eventually you’ll get the feel for that pitch. And I would suggest dropping that over-the-top delivery, because it places your arm in an unnatural position, too much twisting, which can only lead to arm problems—the 3/4 arm slot will be much better for you and will give you more versatility with all your pitches. 8)

I throw my slider with my index finger next to the seam instead of the middle finger, so the seam lies in-between my middle finger and index. Its turned into a habit, and I found out for me its alot more consistent than using a middle finger seam grip. Experiment with different grips, and as Zita said, roll your hand. I found that the most useful piece of advice when I learnt my slider.