It may happen due to many reasons, it may be that:
In order to find out what exactly happened you need to switch your digger to debug mode, run it and check logs. It usually should give you answer whats happened. Or you can always contact us and we will be glad to assist you.
No there is no any limit, you are limited only by number of requests and bandwidth.
We do not charge you in resources when you run your diggers in debug mode. Its done to deliver you free education and tests of your diggers. So we are limiting scraped data usage in this case. To get data you need to switch your digger to Active state and run it in production environment. Then you will be able to download all data or get it using API.
You can run all active diggers you have at same time, we do not limit you. But you cannot run same digger multiple times simultaneously.