One of you alluded to the fact that anyone can pass given enough time. Well the short answer is we don't give them eough time. We place increasingly high hurdles along the way.
At Nav school the process was quite simple. For the first 5 flights each was practiced in the sim twice. If they reached exercise 6 there was a surprise - only one sim. Then the flight itself involved a no notice diversion to a strange airfield. If they passed 6 they usually passed 7 and 8 as well. 9 was really a no-fail sortie at the end of basics.
Next was a switch to (F)J on the JP. No matter whether they were to be streamed FJ or ME they all had to pass this hurdle. Then on to advanced nav training. Even at the final nav check they could be chopped.
We had one wg cdr who bent the rules and let the guys have more than 2 stabs at each hurdle.
Once out of nav school the OCU, esp the F3, demanded a more or less straight line improvement on the grounds that guys who were slow learners would be cannon fodder when the S hit the F.
The aim of the game was to ensure the wasteage ooccurred at the cheapest part of the training pot.