Per an NYT story, the usual suspect:
The Pentagon, in an official assessment of the Afghan mission released last week and current to the end of March, said that “one of the most significant challenges to the growth and development” of the Afghan security forces was the shortage of trainers.
So the US is forced to surge an additional 850 trainers to go with the 1,500-or-so provided by allies. This is beyond the 30,000 troops surged previously.
That's a telling stat, is it not?