Look in the manual and make sure that your LiPo Low Voltage Cutoff is off. Your LED light should be steady red when the ESC is on (that means the cutoff is off). I ran into the same thing.
On a similar note though, I just bought my Velineon and it seems like if I rolled the Slash, it would glitch and not respond to throttle inputs. I would just have to keep trying and then it would kick in. I also noticed that if I would nudge the car, it would kick in. Once it would start responding, it would work fine for a while. This was intermittent, but as I said earlier, it seemed to be happening when I rolled the truck.