it all depends on the settings I think.
A student should be able to resume a chain if the chain settings have "Allow resume" set to "Yes" or "Force" and the either the chain allows unlimited attempts or the student has not yet reached the maximum number of attempts at the chain.
When a student starts a new chain attempt, all previous attempts are set to abandoned in order to prevent accessing two attempts at the same time.
Task attempts work in a similar way to chain attempts.
The default setting to "Allow resume" appears to be "Yes" for chain attempts and "No" for task attempts. This is because it is not possible to resume attempts at Hot Potatoes exercises.