Every time I run a batch, 99% (rarely, 97%) of the tasks are completed, and then the queue gets stuck. Each HIT is being given to three workers. I have paid in full in advance, so it is not stuck on payment.
HITs are likely being dropped if a user writes in a message in feedback or some field. MedCrowd had that issue and so did I recently when I set up a 3K HIT batch to get some data mining work done.