Trying to save a "placed signature" using Acrobat Pro XI, getting a message that states "form is read only or opened by another user"

I have tried this on 3 different computers and I am only having the problem on one of them. The other two are able to open the form with the same Acrobat Pro XI, and save it just fine. This one computer is unable to do such. I have double checked to make sure the form isn't opened by another user or in a read only format. Please help!