"Back" and "Options" buttons are not responding.

Hello, I had to completely replace the screen on my Galaxy S4 T-Mobile version, so I was following a video tutorial from iFixit.com. After completing the procedure, the screen itself shows everything and works perfectly, but the 2 touch buttons (to the left and to the right of the main physical button) are not responding. Can someone please suggest a solution for fixing this, and what could have gone wrong? Thanks ahead!