A couple of other things are also at play:
1- Random cross training pops. About 10 percent of training goes to a random skill. You can be getting unlucky and having these added to low remaindered skills since all skills are graded to the hundredth degree... so your players are getting better, you just can't see it... yet, and these might be in skills that have little effect on DMI even.
2- Also, elastic effect. The higher a skill goes beyond several others (there are debates on hich skills effect which) I personally am trying to keep them all as close as possible until I am ready to finish my builds off. But if you say have OD trained several points higher than everything else, then Pressure training will be less effective but your players will still be improving as well, just slower than they were before when you were training Pressure. This of course can be exploited by training lower skills as well (a common one is training up PA after HN/DR is higher... I am not an expert on this subject but have seen this work firsthand on my 7' trainee.)
Age, height and all that other jazz of course plays in as well.