[AlexITC]

My experience is the opposite, when people is unmotivated or not interested in the work, its unlikely for them to improve even if you give them enough chances.

I have the impression that people get in some kind of comfort zone when underperforming that's psychologically hard for them to improve until they move to another team/company.

So, you should catch these issues as soon as possible to let people know that this is not tolerated, in some way, this helps because people can take you more seriously.

[toomuchtodo]

Could also be burnout.

[pawelduda]

I think it could be just bad fit. Work looks good from outside but once someone is hired, they get disillusioned because it turns out they don't actually enjoy business domain or whatever else and find the job unfulfilling.