Taking previews in Wc3Viewer...

I'm trying to make some .avi previews of a few custom animations I've made. The problem is that all of them are cut (it seems randomly) before the animations end.
Most of them last from 1 to 2 seconds, but I couldn't get a preview longer than half second. Is there some specific action that I'm supposed to do? Also, are there any other programs able to take preview movies?

Thanks in advance.

I have the same problem every time. I just downloaded Camtasia Studio, the demo version, from download.com. It works, but unfortunately my 30 day trial is over : (