The reason that there is a shortage of instructors is that ATCO's want to stay in their environment, be that Tower/APP or Area. They do not want to go to the 9-5 Mon-Fri routine where you only get the weekend off. The perks to working shifts are way superior to working 'days' so the only way that any ANSP can get Instructors is to pay them more, and sometimes even that is not enough. Hence a shortage, worldwide.