Can't set Skim as default pdf reader

I recently updated from Snow Leopard to Mountain Lion. I needed to download a pdf from a government site but when using my default pdf reader, Skim, I got an error message saying I needed to use Adobe, so I switched my default to Adobe Reader. When I was finished I tried to switch my default back to Skim by going into get info on a pdf file and setting it to Skim, then open all with this app. When I clicked out of the application field after choosing Skim the field kept popping back to Adobe. I was able to change the default to Preview without a problem, but I can't get Skim to stick. I can still open files with Skim by choosing "open with", but they revert back to Adobe or Preview icons when I close them. Skim does not become their default. Any suggestions on why this is happening and if it can be fixed?