This may help others where there is no option to Enable/Disable the history anywhere in Task Scheduler.
Open Event Viewer (either in Computer Management or Admin Tools > Event Viewer).
In Event Viewer make sure the Preview Pane is showing (View > Preview Pane should be ticked)
In the left hand pane expand Application and Service Logs then Microsoft, Windows, TaskScheduler and then select Operational.
You should have Actions showing in the preview pane with two sections - Operational and below that Event nnn, TaskScheduler. One of the items listed in the Operational section should be Properties. Click this item and the Enable Logging option is on the General tab.
My problem was that the maximum log size had been reached and even though the overwrite old events option was selected it wasn't logging new events. I suspect that might have been a permissions issue but I changed it to Archive when full and all is now working again.
Hope this helps someone else out there. If you don't have the options I've mentioned above I'm sorry, but I don't know where you should look.