And clean the fiber connector ends with a special cloth or cleaning liquid. Once, I had an intermittent link due to dust particles.

Fun fact 1: When you don't have a proper OTDR fiber tester to check for breaks, you can shine a laser pointer down the fiber and see it projected out on a sheet of paper on the other end (you might have to call the person on the other end).

Fun fact 2: If the other end is plugged into a transceiver, don't look at the fiber end directly; instead, look at it through your phone camera. If all is working correctly, you should be able to see a faint purple light out of one of the strands.

P.S. always remember to roll it if no link initially :)