The tasks often have a limit as to how many you are allowed to work in a single run of the task. This is to allow a variety of works to work the task and ensure the results they gather are from a diverse enough group of people. You are allowed to work the task again when they run a new batch, but it might take a week or so. When you see the task run to zero and disappear, most likely the next time it appears about a week later, it's reset as a new batch and you can work some more. If you get the message "thanks for you work", that means the batch you worked is still running and just move on to another task.