Editing is disabled when there's a reply to your comment. There might be other cases where editing is disabled, but that's the only one that I know of offhand.
Oh! Thank you. I looked for a pattern but failed to see that one. I can still edit my posts after there have been replies, but not my comments. Not even, apparently, my own comments in a thread of replies to one of my own posts.
AFAIK editing your own comments is possible 'til someone replies them; after that, you shouldn't be able to edit them. There was a bug-thingie some time ago that allowed to do so, but now it's fixed.
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