I'm not 100% certain but I think the reason you never see shotblocking messages is because the messages refer to practice categories, not attributes. Since defense practice covers both the defense and shotblocking attributes, you shouldn't ever get a shotblocking message. I looked through my mailbox and I saw none even though I know some guys were maxed.
Likewise, you don't see messages to stop practicing stamina, ath, or speed. it would say "conditioning", and only if all 3 are maxed.
Of course if this theory is correct then he is maxed in everything defensive practice covers.