It is impossible to vote to close a question once a bounty has been added to it. If the question has existing close votes when the bounty is added, they are usually doomed to expire while the bounty is active.
The close voters cannot re-vote when the bounty expires. This can easily kill the momentum to close a question. Ideally one of the voters would flag a moderator, but people are more reluctant to flag than to vote, and this requires that they somehow notice the question has been bountied.
I suggest that close votes not expire or bountied questions, or at least that they be removed when the bounty is added so they may be recast when it expires.