Maybe unbolted just a tiny bit perhaps and the bbls open a few .000.
The top lever may look in the correct position at a glance. But if the front arm of the Fox hammer strikes the cocking slide/foot when it drops before it can go full down, it restricts the hammer from hitting the primer.
You'll still hear the hammer(s) drop, the firing pin tips won't quite reach the primers though,,or will with very little energy left.
Effects both bbls at the same time as there's one cocking slide working both hammers
Close the action smartly and with some authority and the problem goes away in these cases. Again the problem goes away as it came on, effecting both sides at the same time.
It doesn't have to be slammed shut, but you do need the Fox closed all the way for this reason.
Too much clearance in the fired position and they get hard to open.
Check to make sure the toplever isn't bent downward and hitting on the top tang upon closing interfering with complete bolting at times.
They are easily bent w/ your hand or even finger pressure on some and many do hit the tang.
Might not be it,,just a thought.
About the only thing that quickly came to mind that didn't involve obvious broken parts, ect..