I have encountered a few times accepting HITs for crowdsource that suddenly change to saying there are no more available. Why do they keep them posted if they aren't available anymore? Is there a way to avoid this? Also why does crowdflower only let you work on a HIT once and then block you out from accepting more of the same?
I've encountered several hits like that, not just from CrowdSource. I figure it happens when the requesters don't take their hits down soon enough after they get enough responses.
There are more HITs available, just not for you to work on. CrowdSource's Google bomb HITs are limited to a certain amount per worker so you don't waste their money by repeating the same search query from the same IP address. CrowdFlower limits you to a certain amount of HITs due to their usage of 'Gold' units.