Thank you for your post, and I apologize for the late reply.
What version of Adobe Reader do you have installed on the one Windows machine that does display the signature? What version of Adobe Reader do you have installed on the two Windows machines that do not display the signature?
What versions of Windows are you using on the three computers?
Are all computers on the same local area network? Or, are all three computers accessing the file via a WAN?
Are all three Windows computers using the same version of FileMaker Pro?
Any other information you can provide may be helpful in narrowing down the possible causes.