One of the things that this compiler does that can be very exasperating is that if you use in LaTeX a word that is a key word for the site, the line of LaTex in which the key word first appears does not render in the post, but does render in the preview. Example
\(\displaystyle mean = \dfrac{\displaystyle \sum_{i=1}^nx_i}{n}\) renders perfectly in preview, but not at all when posted.
Note the word in blue: it is a key word for the site.
Say the exact same thing later and you get what you expect: \(\displaystyle mean = \dfrac{\displaystyle \sum_{i=1}^nx_i}{n}\).
This was discussed a year or so ago and has something to do with the sequencing of actions in the posting routine. The solution (workaround is better) is that if you must must use a key word in LaTex, figure out some way to use the key word in plain text before you use it in LaTeX. The diagnostic of this situation is that the LaTeX renders in preview mode, but not when posted, and the key word will show up in blue in what you thought was going to be LaTeX..