The most common reason for the buttons to not work is the page didn't load up all the way (takes a second or two here) or there's a Adblocker active.
For some reason when I click the reply button in forums I get no dialogue box which would allow me to participate. This also occurs when I click on some internal links on pages. I am a registered member and have logged in.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic