I don't know how different the W124 housings are from the W123, but those metal rails that run along each of the bulb receptacles can get unseated from the plastic track they are attached to...just enough to lose the ground connection and cause one or more bulbs not to light up.
An easy test is to remove the housing and press down along the entire perimeter of the metal rail with your thumb and see if the bulb comes back on...
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car