Once upon a time, many years ago, ATCOs would be known to do extra hours work or whole shift swaps at short notice for little more than the warm feeling it gave us to be one of the 'hands on deck' keeping the system going. In those days one could also easily swap shifts with a colleague or get someone to go in and work one's shift for the going rate (about £3 2s 6d wasn't it?). Then goodwill started to go down the pan coincidental with having to "work as a business", so rather than working for goodwill or a few hours overtime, it was decreed that we would take a non-pentionable lump sum for attendance. People fought each other to take the dirty money and were quickly working up to the maximum allowed and goodwill continued to be a thing of the past. Large sums would be needed to get anyone in to work outside core rostered hours. Now when someone has got their jealously prized AAVA is it any wonder that they are unwilling to have the goodwill to give it up because management have changed their minds?
The only real answer is to have appropriate levels of staffing and do away with the corrosive AAVA agreement before the rot sets in permanently.
Sadly, back in the real world, too many have got used to the extra cash and will always say yes to more and there have never been realistic staffing levels as management always aim for just enough and not sufficient to provide a hedge against other ANSPs poaching our underpaid staff or medical losses, suprise retirements (it must come as a REAL shock to management that someone with 40 years service could be interested in hanging up the headset).