When I worked in the UK at one of the larger units the success rate was 2 in 3. The system used to be quite ruthless but qualification was made a bit easier by training on only one pair of sectors rather than two. Eventually the 'tick box' training system pretty much allowed as much time as was needed unless it was clear a trainee was completely clueless. This did leave the OJTIs wondering how the person had been allowed to make it that far!
Some of the trainees were very motivated and able but for quite a lot of them ATC was just a job. Their attitude was that if they didn't pass they would just try something else The motivator for most of the trainees I encountered seemed to be the comparatively high salary rather than any interest in aviation.