What version of RE:Flex are you using? You should be using version 3.0.5. You can find out the version by clicking on the "About" button when the RE:Flex operator is selected. If you are using combustion 4, then make sure you are using the latest version, which is available on our website here:
http://www.revisionfx.com/rflxversion.html
If you continue to have problems, please send us a short source clip and combustion workspace with RE:Flex Motion Morph set up, and steps to reproduce the problem. You can send the stuff to techsupport@revisionfx.com
Pete LItwinowicz
http://www.revisionfx.com