When I turn on Progressive Caching on Joomla 3.1 the modal popup stops popping up. When I turn off caching it pops up properly.
Unfortunately cant see an option to turn off the cache for the module.
I would really like to enable cache because my site is a bit slow without it.
First, I'd recommend "Conservative Caching" over Progressive unless you know what you're doing. Progressive is actually more complex and slower than Conservative, though the name and ordering make it seem like it's the better option.
If it still doesn't work, let us know. The caching options will be in the next release as well, but that may not fix the problem either due to the way that we insert the modal popup onto the page (technical reasons, and something we're looking to also improve).
Finally, also check if you're using the System - Cache plugin. If so, try leaving Global Caching at Conservative (or even Progressive if you want) while disabling the System - Cache plugin.