Saved the project. Closed out of every program that was running. Turned off the computer, waited 60 seconds, turned it back on, then only opened Camtasia and then tried producing. That seemed to work for me every time.
Another note is I have found on occasion when using b-roll from another source (usually free), even if I was able to put the b-roll video on the track, when it came to rendering, it would not render. There was something about the codec of that particular b-roll. What I would do then is take the original b-roll, put it into free conversion software such as Freemaker Video Converter and convert it to a MP4 format. I would do this even if the file was already MP4, reconvert it. I would make sure to save it under a different name. I would then replace the original b-roll with the converted b-roll. That usually did the trick.
Polar, This can occur if there is some corruption at the point at which the production gets stuck.
Take a look at this thread for methods on how to correct. There are even formulas for converting 44.5% into time to find the issue on the timeline. Often, usually?, playing video starting just before the corruption will show the exact point the corruption starts. Save the project first!
Oh, if this doesn't help it may be another issue so let the community know and consider contacting support.