Stamina decreases ever so slightly each week. And if the next week after they hit 10, it will drop and show as 9 (which in reality is not much different from 10 because it's like 9.9/9.95). From what I have gathered from extensive reading in the forum is that in the week you are expected to drop a skill, any training will be negated by the drop. (Thanks to some very intelligent observations by BB members who I have now forgotten their names)
What I mean is the training will not count, and it will drop as if it never received any training at all. So even in weeks you are training stamina, don't complain if it drops. Many managers have been complaining about this already.