There are often times when I install an addon (place the file/folder into my blender/2.XX/scripts/addons folder) and then it simply does not show up in the addons list within blender, so cannot be activated. Installing it from a file places it in the blender scripts folder in my app data folder (windows 7), but they still don't show up. These are addons that I know work for others, as I see them install them in tutorial videos or the like (.py, not patches or anything.)
So far, I have checked to make sure I am not filtering them out with the Support level filters.
What causes an addon to not be seen by the program, and what further troubleshooting steps I can take to get these working?