IF your bulb is good, AND
IF the contacts are clean, AND
You're measuring 12.8V at the bulb, AND
You still have no light, THEN
You must have a bad connection somewhere in the circuit. It's enough to carry the miniscule amount of amperage needed to activate your meter, but not good enough to carry the amperage needed to light the bulb(s).
The problem could be on either the ground or the positive side, or both.
Check your connections, beginning at the battery
, then the CPD, the panel, any subpanel, etc.