Ones with big blocks?

AMNESIACS

Think of people who might struggle to recall large chunks of their past, especially after a head injury or medical condition. The answer starts with a letter that also begins "amnesia.

This clue plays on the double meaning of "blocks." In one sense, it hints at "mental blocks"—when someone forgets things, like an amnesiac. The other meaning is literal big blocks, but the wordplay tricks you into thinking of something physical before realizing it’s about memory loss. "Ones with" just means people who have that trait, so "amnesiacs" fits perfectly.