It sounds like they have little skin in the game. Does your org reward hard work and hitting targets? Is the company doing mission critical work or feature farming? I dont think there's a company I've ever worked for where the status quo wasn't a full on regression to the mean average of the culture of the place. Conway's law? They were hired for a reason but no one can be firehosed with bullshit for long, before simply giving up if there's no compelling reason to continue striving.

So, have you spoken to them? Or, put them in a position where there failure to deliver will have real world negative consequences to people on the team they care about?

For me personally, I seem to only operate well under intense pressure and heavy load. Maybe they're bored. Maybe there's confounding variables outside of work they need to put time and attention into before they're unblocked and consistently in a high delivery flow state. (Or, perhaps, you need to tell them: we want you to try harder to fail.)

Edit: also, work expands to fill time. Intellectual and creative work accomplishes things in leaps and bounds or fits and starts IMO. Punching a time clock is terribly draining... Is there a shared team vision?