I found an issue with Yosemite on one of my ssids.
I had max-tx-fail set to 25 (I beleive a pre client-match way of dealing with sticky clients)
the default is 0 (unlimited fails)
returning max-tx-fail to 0 - appears to have resolved the issue. At least for my macbook that I was forcing into the problem state. I only just made this change today and await feedback from the users.....
.... so ..... yeah....
Please see if this is pertinent to your problem ssids and test and report back as well.