Mail signatures are identified through the use of UUIDs (Universally Unique Identifiers) which are associated with your various email accounts through a property list called 'ist'. Still not working? Delete signatures plist
Then set each account's default signature. To do this, go to your 'All Signatures' list and drag the ones you wish to use to their respective accounts. Additionally, while signatures are done on a per-account basis, they must first be associated with an account before you can choose one as the default.
This may not be apparent to some users, especially since Mail initially shows the 'All Signatures' list with one highlighted. There is no global setting for the default signature rather, this is set on a per-account basis in Mail. In order to use this menu, you must first select one of your email accounts and then set the default signature for that account using this menu. The 'Choose Signature' option appears to be nonfunctional and set to 'none.' You set these up in the 'Signatures' section of Mail's preferences, but when you try to set a default signature to use, the 'Choose Signature' menu is grayed out and set to 'None'. Many people have a number of signatures they like to use with various email correspondences and email addresses.