Finnprog, one reason holds occur is because the students are too good, the instructors are too good and OASC was too good.
It happened back in the 60s too. The nav schools had a given number of slots per course. The into ITS expected some failures. The basic nav course could then take 18 per month with a proportion of chopped pilots and the rest straight through navs or siggies converting.
The next nav school was established for courses of, IIRC, 14 which included Cranwell basic navs coming for the advanced phase.
It followed that the basic nav school had to reject 5-6 students per course. After much humming and harring they back coursed increasing numbers in the basic nav school. All but one of our course was put back one month to start again and higher up the tree fewer and fewer were back coursed until the system was sorted.
As an aside, the one who was not back coursed passed out on time, went through the next 2 schools and OCU only to get killed within a month of arriving on his sqn.