Hiya! At the bottom of their post, on the right hand side, there should be a few icons, one of them being a 'thumbs up' with the word 'Thanks'. Click on it and Bob is your proverbial uncle!
It's on all the above posts. where have you noticed its not on? Of course, its not on your own post cos you cann't thank yourself, is this what your noticing, that its missing off your own post?