Why can't I turn on the display sometimes?

I have recently changed the display on my SE 2016 Iphone with a brand new one and now sometimes when the phone is on sleep mode and I try to turn the display on by pressing either the home button or the power button on the top, it does not react in any way, the screen remains black. I usually have to repeatedly press the home and/or the power button to be able to turn the display on. This only happens about 30% of the time. Most of the time it works properly. I thought I might have not pressed the display flex cable properly. I have opened the phone again and connected the flex cables again but it did not help. Could it be that I damaged somehow the display flex cable? Or it could be a software error?