I've currently placed a bounty on Math.SE (First bounty I've ever placed if I'm correct) and wanted to place another on SO but couldn't, I've tried looking at few questions all older than 48 hours (some frequent/unanswered/hot including my older questions) and none had the "start a bounty" button. I've checked both IPS and Astronomy also and couldn't place bounty. I've also tried to place another on Math.SE (but couldn't).
From the tour/help page it says that an User can have at most 3 active bounties so I'm baffled since I only have 1 active bounty.
I've found the link Missing "start a bounty" link on stackoverflow perhaps this has to do something with me having an old account and never casting a bounty before?