Very good question, Nick! I see this too very often and I keep wondering how it’s possible. I’m curious about the answer.

I used to explain this by fractions of skill points. A player can gain less than 1% for a particular skill. I that case you don’t see it, but when the skill increased from - let’s say - 98,8% to 99,1%, the visual effect is a gain from 98% to 99%. This may result in a shift from 99% to 100% overall and an increase of a star.

At the start of the season I counted the visual skill increments of my players while power training them. The sum of the % from the training results was exactly the same as the incerement of the player’s skills on the long term. So it seems there is no such thing as skill gain not being shown from training results. Therefore, my answer seems to be incorrect.