That is a bit of a puzzle for me, too. It may be an issue that just cannot be diagnosed over the 'net, and will require hands on inspection by a good double gun smith. I will be thinking about it, tho' I make no claims about being sharp on such issues. One thing I would do, if I could not see an obvious issue with the triggers upon disassembly, is to compare them to another Fox that has perfectly functioning triggers. Sometimes a side by side comparison (pun intended) helps me diagnose a problem.
One thing I would like to confirm, were it mine. That is, will the same thing occur if you fire the left barrel first, then try the right? I often shoot my doubles that way, if the barrels are choked differently, at clays or at birds. How much trouble would it be for you to try that with the heavier pheasant loads you were using? I could shoot them off my front porch, but I know everyone doesn't live as far out in the sticks as I

. Another question, when the rear trigger will not fire, how does it feel? Does it feel "blocked" from being pulled, or does it feel "spongy", etc.?
SRH