Sometimes it's the sensors. If they're not very responsive (to me, when pressing each bracket down, if I have to apply more than, like, 2 pounds of pressure, then it's not sensitive enough), then you'll need to replace them, as they may register your step late if you hit it on certain parts of the pad.
Try cleaning the sensors up. Make sure there's no dirt caked around the sensors (we're not looking at clean-enough-to-eat-off-of, but just so there's at least minimal visible dirt), and if you want, unscrew the brackets and wipe them down (if people spill coke or anything on the pads, they can get sticky). Also, make sure the spacers and screws are secured relatively tight enough so the brackets can move freely, but not so tight that you'll never be able to unscrew them again.
Stages I think can be bought separately, but they might be insanely overpriced. Try going through Channelbeat or Betson for sensors, but only if the sensors are mostly dead or dead, because those sensors are quite pricey (CB, you have to buy in bulk). | 
