Can't find a user in all collections when trying to add to a permission group

Question

I have several site collections on SharePoint 2010 and a user that can't access them. When I try and add this user to my visitors group I am able to add them to 2 of the 5 site collections that I have. The others I get the message "No results were found to match your serch item" I have verified that this user is part of my active directory and I have no issues finding them in the 2 collections. I have tried running a powershell script that I found: Set-SPSite -Identity http://mybrokensite  -UserAccountDirectoryPath "":   then running a full profile sync but this didn't seem to help, the user still can't be found on the other collections. Any ideas?