I want to place a bounty on this question but I think I already did that some time ago. Is it why I don't see the "start bounty" link at the bottom of the question, despite the question being indicated as "eligible for bounty"?
You cannot currently offer a second bounty on that question because you do not have enough reputation to do so.
The minimum bounty amount doubles on each subsequent bounty you offer on a question:
Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on).
Since you already offered a 100 rep bounty on that question, your second one would therefore need to be at least 200, and you do not have that much rep to offer at the moment.