Touch screen works only when I press motherboard

Hi, I've got a problem with my S3 LTE. I have changed full set in my phone. Connected it to the motherboard and there is a problem with touch sreen. Once it works, then it suddenly stops and doesn't work. I tried to change ROM, I have contacted seller of set, because thought that is faulty. He sent me a new one and it is the same problem. When he got the package with that first one, he wrote to me that it works perfectly with his phone. After few weeks with this problem I saw something new today. At the time, when I'm pressing motherboard in red selected area, touch screen works! It is not about the LCD flex, because when try to push this flex it doesn't help me. It works always when I press here, like on image. What is an issue? What could I do? (sorry for my English, it is not perfect)

Here on this picture it is i9300 but it's the same place as in my i9305.


Thanks!