The problem is that some people have to high expectations for kids like Malaki, Blake, Tre or Keldon. They are what you'll mostly get for those kind of picks, average role players if you're lucky with some potential but also a lot of flaws, then end of bench to borderline NBA players in many cases. Fans tend to focus on the potential, hoping they'll get rid of their flaws.
But there's a reason they"ve been drafted in the 20s or second round. If they had any kind of real star/elite role players potential they'd have been picked higher. And being drafted by the spurs changes nothing to that. Yes, they'll keep improving, like anyone does pratcicing at something, but they're not underperforming or underachieving so far, they are who they're meant to be, this is who they are, non lottery picks. So we shouldn't be surprised, disappointed or put too much hope on them becoming more than that. That can happen but much more often than not, it won't. Blake or Malaki may even be out of the NBA after their rookie contract.