I had exactly the same problem with the ExtraContent stack. Fixed by reinstalling.
Latest version of stacks, rapid weaver etc.
Since then however I have had further problems. I purchased the Potion_Pack for Foundry, double clicked on the install icon and received a message that installation was successful. The stack was not however available when I restarted RW, nor was it present in the dropbox folder chosen for add ons in RW preferences. There was however a folder for foundry_potion_pack in the application support folder of my library
I solved the problem by dragging the stack directly into the appropriate dropbox folder but is there an underlying problem with the default location for installation in the latest versions of the software?
Many thanks for any advice