Interesting, I'll upvote you and see if that sticks.

yes, that one did work.

This is weird!

It looks like it is related to where you are in a thread relative to the things you comment on.

So if you haven't been active in a thread yet you can comment on anything, but once you are active your voting options are limited, but it doesn't show that by removing the buttons.

Yes, it seems once you're responded to someone, you can no longer vote them up and have it stick.

Cool, thanks !

So that means it is a display bug rather than a voting bug.

Those particular 'up' arrows should simply not be there.