Michael,
Make sure you're running JFBConnect v4.1.1. That fixed a few issues with the Joomla Cache plugin. Other than that, we test pretty heavily with and without the System - Cache plugin, so it should work. As always though, the Cache plugin should always be the last ordered (highest number) plugin in the system group.
If that still gives you issues, can you explain more by "sometimes doesn't work"? Do you get an error? Weird redirection? The button doesn't seem to do anything? Any information you can provide will help us give the best solution we can.
Thanks,
Alex