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Can you not reply at all, ever? That's sometimes a javascript permissions problem -- check to see you're not blocking scripts from running.
If you're just sometimes unable to reply, that's most likely a symptom of the problems the site as a whole has been having lately -- people have been reporting that sometimes it's hard to get a reply window to open. I generally find hitting 'report' and then 'reply' again helps with that, but sometimes it takes a few tries.