There are several factors that may cause a bottom barrel on an O/U to miss fire .
Dirty
broken firing pin
piled up firing pin return spring
deformed hammer and/or firing pin
firing pin hole deformed
weak / broken hammer spring
The geometry of the bottom firing pin is lower on the pivot point which the nose of the firing can get swept by the barrel upon opening if return spring not working properly .
The hammer has less arc on the bottom firing pin than the top firing and everything being "right " is more critical on a bottom firing pin than the top.
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