Vlahovic maybe, but Chiesa had a period while he was on fire (that pre-injury run of form). So it's not that he doesn't under Allegri but the guy had a bad injury, and I even said that I highly doubt that he wil be the same player ever again. You could see that he lacks confidence when he gets physical or whatever. He isn't as sharp as he was earlier.
Vlahovic's situation is different. He was a 6-month wonder at Viola. It's always tricky with these huge transfer prices. Expectation gets higher automatically once players are sold for crazy prices. Milik was brought as a low-profile signing behind these BIG transfers at gobbi, but the guy managed to bang in some important goals before their entire collapsed because of various reasons.
I really think Allegri isn't that bad. If you consider all the circumstances around gobbi, in and off pitch then Allegri hasn't done THAT bad. Just imagine if any other coach whom is not as authoritative as Allegri, what would they have been going through all these things. It's not like we care about it, but would another coach solve gobbi's problems? Of course not. They are constantly rumored with these scandals, which surely affects mental state of the team, then issues in the management, transfers policy and everything else. They have some real issues though.