If your ability to touch on the screen doesn't seem to be working so well on your Pax payment terminal (S300 / Px5 / MT30) there's an option to recalibrate it.
1. Unplug the terminal and plug it back in.
2. Hold down the menu button when it appears on the screen.
3. Choose the calibration option from the menu that comes up.
4. Go through the calibration process, you will want to use a stylus to ensure a quality alignment.