I've seen this happen from time to time if I'm rendering to AC3, where if I try to catch the same file name twice, it won't render but a short segment. If you're rendering to AC3, try rendering to a new file name, see if that fixes the issue. It does for me.
I've gotten in the habit of whenever i render AC3, MPEG, or MP3 over top of a previous existing file, i always delete the previous file first. Only takes a few seconds and seems to avoid a lot of grief. I started doing this back in Vegas 4 days when i discovered that if you cancelled an MPEG render early and then redid the render to the same file name, the new file would stop playing at the spot where the first render was cancelled. *shrug* Very weird.