I've noticed this more than once and thought I'd document this quick tip for the community. Basically, I can't stress enough the importance of praising and criticising training performance. For the former, I try to only do it once a fortnight for each player rather than every week (to avoid nonchalant responses), but I've occasionally noticed free stat boosts from this process.
This is Jack Evans who has done okay in this week's training:
Look what happens when he is praised:
Nice little attribute boost for minimal input