Found this solution in another thread
The answer was to go to "File ->Export" and save as a new PDF. There's something about PDFs that are generated from a certain version of Acrobat which have this problem.
There's an even easier solution to this very frustrating problem. Open your original pdf in preview, then choose "file - > duplicate". Then add the signature in this second (duplicated) version. For some reason the original pdf will rotate the signature but the second (duplicate) version orientates it the right way.