At work where cable runs are usually made by maintenance people the most common problem is poor termination. They often just crimp a connector instead of using patch panels/sockets and unwind too much of the cable before connector which causes all kinds of problems. With proper termination problems usually go away.

But it can be a ton of other stuff too. Good cable tester is pretty much essential to figure out what’s going on. I’m using 1st gen version of Pocketethernet and it’s been pretty handy, but there’s a ton of those available, just get something a bit better than a simple indicator with blinking leds which can only indicate if the cable isn’t completely broken.