A.T. Perry waived (Unclaimed off waivers); Perry signs to the Broncos practice squad

Well, allegedly, his problem is he didn't learn KK's offense, which would explain at least in part why he was 6th on the depth chart. And probably why he chose Denver because he knows their offense.

That said, Allen could simply sign him back to the roster next week if he wanted to, but I don't see that happening if he doesn't know the offense.

Regardless, some players don't work out. It happens.
Where did you hear/see this report? It seems that every time there's an intriguing udfa or middle-low draft choice, the reason they're not playing is because of their inability to learn the playbook.
Long ago we had a back named Wilmont Perry. He was tough and somewhat reminded you of Eddie George because of his style. The "rumor" why he didn't play or last is because he couldn't learn the playbook.
Well you have to be brainless if you couldn't learn Abramowicz's high school offense.