Its called throttling. mTurk stops people from hoarding/slamming heavily active HITs 'cause reasons (probably good-decent ones, but they don't tell us shit so
). Just panda them in PC/MTS HC. They'll trickle in but do them from queue as quickly as possible.
Blocks on worker will throw up the "You are not qualified" message if there are HITs available but you're blocked. If the HITs aren't active it seems to revert to the "None available" message but truthfully I haven't seen super consistent behavior on it to know FOR SURE that's why it reverts