Add-In says "Missing" - DLL did not install

Hello all,

When my 10.1 Add-In is installed on a client's machine, it does not show up properly. It says "Missing" on the toolbar and clicking the button does absolutely nothing. When I check the user's AssemblyCache, I only see the .pbd and .xml files. The DLL isn't writing. I'm not getting errors messages of any kind. It works on all my office machines.

It doesn't work using either the network share or local install methods.

Any help is appreciated,
Corbin de Bruin