Wrong button. It's L2, and you have to make sure you time it right (letting go at the right time) or you'll be getting thrown out or picked off quite a bit. You can also just push the button once to queue the steal, and the runner will take off when the pitcher starts to make his pitch. That's how I do it. If I know I'm gonna steal, I'll push L2 as soon as the batter steps in the box (your controller will rumble when it registers the steal command). This is ideal for Hit & Runs also, as it lets you focus on the pitch instead of getting the timing right with sending the runner.
Also, be warned that only using L2 is global, so say there's runners on 1st and 3rd, then just pushing/holding L2 will tell both runners to steal. Say you're in that situation (1st and 3rd), and you want the guy at first to steal, you want to point to him with the Left Stick and hit L2. So you push right to indicate the runner at 1st and L2, and he'll steal, while the guy at 3rd will stay put. Same thing if it's 1st and 2nd, if you just want the guy at 2nd to steal, you'd push up on the LS and L2.
Also, you can also take a bigger leadoff with the L1 button (same principles as stealing, with it being global unless you point to the runner you wanna lead off). Each time you press L1, the runner(s) will take an extra step leadoff of the base. This you really have to be careful with, as unless your runner is a fast AND a great baserunner/stealer, then you'll get picked off a whole lot. Unfortunately you taking a bigger leadoff seems to trigger the pitcher to throw over quite a bit. So be careful doing this.
I use the extra leadoff mostly at 2nd base, while there's 2 outs, so I can get an extra jump on a base hit. The good thing about doing it while at 2nd is if the pitcher is gonna try and pick you off, you can see the fielder come in behind you so you can hit/hold R1 right away, which shortens the lead and the runner gets back quicker. With 2 outs (and/or with a runner on 3rd), the fielder is usually further away from the base, which make it practically impossible to get picked off. With less than 2 outs though, he plays closer, so you gotta be quick with the R1 button.
