Two fingers touch isn't working

[AC Question]

Sometime when I touch the screen with two finger on my galaxy core prime, one of the finger touch isn't working and sometime it work.

With two fingers on the screen:
-If I press harder it work
-If I slide my finger from a position that working to a position doesn't working normally, this position work without need to press harder
-I have this problem mostly when my two fingers are on the edge of the screen

 

[the tall guy]

have you dropped the phone or damaged the screen in any way? have you taken it to a repair shop for someone to look at it?