Its not necessarily 'failing' the OPQ, it is that however you answered it created an impression that was not suitable/compatible with what NATS wanted, this may have been due to you lying outright, deliberately, or simply that you do not fit the bill that NATS has set out in what it wants from its candidates.
So its not necessarily 'failing', it is simply that the OPQ has ruled you unfavourable or uncompatible if you like for the role of trainee ATCO. Obviously some (maybe the majority) of candidates are likely to be found uncompatible and therefore their application rejected and further progression denied. If it wasn't possible to 'fail' or be found unsuitable upon the basis of the OPQ then what would justify its inclusion in the process? Simply to gauge your character??