The problem generally arises if you reply using the same wording as you have already used in a previous answer, or if you make a stock reply like LOL or even just thank you. It appears to be some form of built in spam filter.
Sometimes you can get over it by altering the wording of your post slightly, even maybe changing the punctuation.
Another way of getting over it is to type part of your reply, and then edit the post once it has been posted.
So when it happens, use your browser back button and alter the text slightly and try reposting.