The attempt is submitted for marking when you click "Submit all and finish".  Once you have done that you cannot try again: each lab quiz can be submitted for marking only once. 

The attempt is submitted for marking when you click "Submit all and finish" but you can try again after that if you want to have a new attempt marked.

You need to click "Submit all and finish" if you want to save the attempt so that you can come back to work on it again later.