Why am I getting "fatal: not possible to fast-forward, aborting" when trying to merge with --ff-only?

I used to rely on fast-forward merges, but recently Git started throwing this error when I run:

git merge --ff-only branch-name

It fails with: fatal: not possible to fast-forward, aborting

I’m aware that --no-ff has its use cases, but I’m confused why a fast-forward merge isn’t possible anymore. The branch hasn’t had any conflicts or manual merges as far as I can tell. What are the common reasons for seeing the fatal: not possible to fast-forward, aborting message, and how can I check or fix it?

Any insights would be really helpful!