Why can't I post comments, sometimes?

I post comments on people's photos--and sometimes it seems they just won't "take." Anybody know why that is?
On my own photos, I don't see any available option to limit who can post. Of course, I see where I can keep a photo to "Friends Only." (I haven't done that--ever.) But obviously, the photos where my comments won't take are not set that way--or I wouldn't be able to see them.

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