I had the same thing happen. I trained 4 players in RB (C/PF, lvl 9/8 trainer) for 8 weeks, and had only one player pop in Inside Shot.
It appears that what we thought we knew about the training of secondaries was highly optimistic. It will be very difficult to see just how long it takes to have a secondary skill pop twice, because the primary skill will increase very quickly (perhaps too quickly). In order to really know the speed of the secondary training, one would have to train the same primary until the secondary popped twice; from my experience that would mean training only one skill for more than a season and creating mono-skilled behemoths in the process.
For instance, I'd suggest an 18-20yo player will pop in RB twice every three weeks (I'd assume this is also accurate for all other skills, but haven't tested it yet) or 7-8 levels per season (not including playoffs). So a player could easily be trained from respectable to wondrous in a primary skill during the course of one season, without his secondaries popping twice.
The prevailing wisdom around these forums has always been to rotate training. I am apt to agree, as longer I trained RB during my 8-week study, the more I saw my team suffering in other areas (i.e., I now have a player with prominent RB that still has atrocious SB).
@WFU03:Perhaps it would be best to remove the suggested # of weeks from secondaries altogether in the first post. For instance:
RB C/PF
Trains RB 1-3 weeks
Also trains IS, ID