Hi Chris, Welcome to the forum.
There may be a problem with the HTML inside your LIT.
Try running a LIT to EPUB conversion with [Convert] - [Debug] on. You will specify a directory to catch Calibre's intermediate conversion files. Ignore the EPUB output.
If you look in the Input sub-directory you should find the HTML that Calibre extracted from your LIT. Open this in your Browser and/or a text-editor to see whether the rest of the book text is present. Try and track down where the problem might be in the HTML.
Once you've corrected the HTML you can re-import the HTML into Calibre (which will zip it up with any attached images). You can then do a "proper" ZIP to EPUB conversion.
Hope this helps.
P.S. I recently had a similar problem with a PRC to EPUB conversion. The HTML had some FF chars (Formfeed???) in there, at the point where the truncated EPUB ended. Once I got rid of them, everything was OK.