I am sorry to say I am not sure why the signature is not working for you in the table. It's really a mystery. Maybe you should try creating a table in your sandbox and adding your signature in one of the cells. Does that work? If you think this is a general problem, then we should report the problem on the WikiEducator Google groups list. It's best to do some trials to pinpoint the problem best you can beforehand. Let me know how you progress on this.