Restarting Performance Equation calculation following a "Scan Off" value

All,
 
I've recently amended the scheduling of a PE tag from event-based to scheduled on a 1 minute basis.  However, this hasn't given the expected outcome, and I've a "Scan Off" value for my latest event (some hours ago, now).  I was obviously expecting a host of events at one minute intervals: what have I managed to do wrong?  I wondered if I needed to prompt the PE scheduler to pick up that the tag attributes have been amended, so set the 'Archiving' attribute to 'Off' for a few minutes, and then returned it to its original setting of 'On'.
 
I'm asking as someone who's been merrily extracting data into Excel using the SDK utility for a couple of years, but who has recently been moved into an area where I need to take more control over data capture and archiving.  Consequently my technical knowledge of how PI works is limited, and I've pretty much reached the limits of my understanding.
 
Many thanks in advance,
 
Rob
 
<Edited for poor spellunk>