Bar none, the best data about training comes from the crowdsourcing project described here:
(203921.1)Join up and you'll be able to see the data and analysis. There's analysis of height effects, age effects, trainer level effects.
just 2 pops in a skill does seem a tad slow, even with two position training. But maybe if you have unlucky starting sublevels, and you're doing something like pressure training on non-ideal height trainees, it might happen. Really it's hard to answer your question without more info. Also, if you're doing two position training but only wondering about two of your trainees, does that mean that the other guys who got training did well?
As far as trainer levels, I do think level 4 trainers get the job done for most trainees pretty well (and perhaps most cost effectively). I use a level 5 trainer, and have been happy with the slight boost in training speed. But now is a bad time in the season to get a new trainer. Wait until the middle of the season, and the prices will come down. I think I got my level 5 trainer with ~27K salary for 50K.