JEP not measuring elapsed time duration between status.

[Karl Samson]

I'm trying to measure the elapsed time between certain statuses for a Sub-Task. I have configured the appropriate Custom field (Sub-Task - Elapsed Time) and also configured the Context for the field, specific to the project and the issuetype. In order to check it is on the right screen, I have turned it on to show when 'blank'. 

Whatever I do in terms of moving a 'Test' sub-task between the statuses, it states that the elapsed duration of the issue  = 'None'? 

Has anybody come across this issue, as I don't what else to try? Running version 7.2.0 Jira 10.

Thanks,

Karl.