Nobody is a mind reader - the FEAST test is designed to single out those applicants that have the necessary personality/cognitive skills identified to make 'good' controllers.
What then happens in the training institute/training simulators is totally down to the individual students and inevitably some don't make the final grade (and this is for far more complicated reasons than 'no aptitude' like the other poster suggested).
The failure rate of 50% is also dramatising - of late groups passing through IANS have been very small, for eg 4 students - and so you can do the math on how many it takes to fail to get a 50% failure rate.