Several solutions exist because their can be several causes to this issue:
- recalibrate your keyboard, be sure not to touch close to the back or home keys. To do this touch and hold the "," key while the keyboard is open then go to Advanced and click the calibration option.
- Do a software update
- Turn off the power save mode
- Make sure the screen cover is not covering the back ad home buttons.
If none of these work call HTC and get a replacement.