Check your profile settings
Your signature must be activated under your profile settings. Go to profile, preferences, and find, "Always attach my signature." Next to this you will see the options of Yes or No. Change this to Yes. Go to the bottom of the page and hit Save. Once you change this your signature will show up in all posts made after this setting change.
Check the posts that do not display your signature
Once you've changed your profile settings, your signature may still not show up in posts made before this setting change. Simply go into these posts, click edit, go to options (which is below the text box), and click "Attach signature (signatures can be changed in profile)."
Your signature may be too large
Try trimming it down to something more simple. If you still have problems, contact an Administrator.