In an HTML enabled Forum if you use the 'less than' symbol < by itself and not as the opening of an HTML tag where it has a corresponding closing >, for instance if you use < as part of a mathematical expression, the Network54 software will assume you are opening an HTML tag.
Everything following the < will be treated as an HTML tag until the software encounters an actual HTML tag or one or more line feeds (presses of 'Enter') or a > ('greater than' symbol), or the end of the post. Since what you have typed until one of those contingencies arises is very unlikely to be a real HTML tag the software will simply dump it into cyberspace and it will not appear in your post.
The best defence against mishaps like this is to use the 'Preview' facility before posting, when it should be possible to spot the problem and work out what has caused it. Unchecking 'Enable formatted text (Huh?)' might also be effective, but you wouldn't be able to use HTML and you might not be able to use smilies.
If you really need a < in isolation it is possible to obtain one by using a special HTML character string which I have used to display the < characters in this post. However you need to be exact. You also need to note that in this case previewing your post will mangle the character string and you'll need to correct it before actually posting. The required character string is:
<font size="4"><b><b><</b></b></font>which will produce a < symbol in your text. The character string doesn't need to be typed in large bold characters, that's just to make it a bit clearer.
There's probably no easy answer to this for the Technical Staff, although as noted I'm fairly sure that it used not to happen.