It is quite difficult to pinpoint the exact cause of this problem, since I have experienced it several times with older version of Revit. Here are some the causes I have figured out and helped me in some of the cases:
1. Specific version of the dll file and its location is important. Some time it should be located in the Revit addin folder, some time it should be in the window GAC folder. I have not found any documentation regrading this issue, just from my own experience. And it might be different from each Revit version. I am still fuzzy about this, any official explanation is much appreciated. So, maybe Revit did not find your dll where it supposed to be.
2. It is the mechanism of Revit to load it library, then read all addin file from all addin folders, sort and load the addins them in alphabetical order. To boost performance, Revit only load each dll file once. Therefore, I think that other version of your addin dll might be loaded before your addin due to native Revit library or other addin on your machine. You might want to search all Revit and its addin folders or write some code to list all loaded library and try for explicitly load you dll in the External Application start up event to see if it works.
Hope this helps.
