Tom,
If there are two bulbs in the front or rear for the same side, one can cause it, but it tricks you into thinking it is working normally.
Also, if the filament is bad, it won't light. However, I have seen many that appear good but are not.
Let me know how things are turning out for you or if I can help.
Take care and Happy 4th!
Joe
Sunday, July 2nd, 2023 AT 1:13 PM