We recently completed a data job for a big customer and when they came to plug up their VOIP 'phones they discovered a problem with the cat6 cabling/wallboxes we installed. After lots of head-scratching and pulling the modules apart I discovered that there has been a batch change - notice the different colour of PCB. The newer modules (yellow PCB) have a stronger spring to close the dust cover (second picture).
Now, when you plug in certain brands of pre-made patch cords the stronger spring pushes up on the RJ45 and forces it back out of the socket by a small amount - maybe only a milimeter. This is enough to make the connection unreliable.
It thankfully seems to be a batch issue as only a few of the thousand modules I bought seem affected!.