I opened a Camtasia project I haven't opened in a few months and an mp4 file included in the project wasn't displaying anything except the audio.  I converted that file from a mov file a while ago and looks like I was using a codec no longer supported by Camtasia after the latest upgrades.

A simple search on the InterTubes led me to this TechSmith article saying to use HandBrake, a free converter tool, to export to a format Camtasia would like.  Another nice tool in the toolbox! :-)