Do you see a flashing cursor while you are typing?
(a cursor is a little vertical line about the height of a capital letter. It is supposed to flash on and off to show you where you are supposed to be typing. IOW, anytime you stop typing, you should see what looks like a very skinny capital "I" at the end of the last word you typed.)
If you are not seeing that (and as I mentioned, I have had some occasions recently where mine has disappeared here on FSM), composing a post can be quite frustrating.
At the end of a paragraph, hit the "enter key" (as mentioned by jtilley in the other thread.