Touch screen sensitivity, only picks up touch

I recently had a cracked screen replaced on my LG g2 mini d620 and it now only picks up touch on the screen. It doesn't pick up multiple touch or sliding/scrolling on screen. I hav been told it may have to do with sensors or something but no idea how to fix it. Any suggestions? i am not experienced with these things but i believe the guys who replaced screen are taking me for a ride