Hi, The TX LEDs are connected in two series strings to keep the current consumption, and therefore the heat, as low as possible.
If one of the telltale LEDs is out then there is an issue with that string - it could be a poor solder connection on any of the LEDs in that string or an open circuit LED in that string.
An easy check is power the board up, using a small flat blade screwdriver short out one LED at a time and see if the telltale LED comes on. If it does, you have found an open circuit one.
