Typically, on the mobile app you will not see signatures. So, that won't fix it. Do you have it checked in your settings to see signatures? Under Preferences, you will find this: Show people's signatures with their messages...make sure it is checked.
i think some one posted a thing a few months ago about needing to use the mobile site in landscape rather then portrait to get signatures to show up.
Mine is not working either. Am I not allowed to link my grow as a signature. I read the rules and didn't read that we can't.