Why the digitizer is not working?

My Nokia Lumia 625 fell down once and the touch-screen was broken. Then I replaced the digitizer and it was working well for only one month. After that the touch-screen did not work. Again I replaced a digitizer by myself and it worked well and good for only one month. Now it is again not working...

The touch-screen is working but the below three buttons (back, window and search buttons) are not at all working. What might be the actual problem in this? Kindly give me a solution...

Thanks in advance.