tdameritrade log on

I've had this problem all along and can't figure it out. I have never been able to log on to my ameritrade account from my iphone. When I enter my log in information, it takes me straight to a screen that says "for your protection, your session has timed out" without ever having actually logged on. I've poked around and tried to figure it out, but no luck. I've got it set to accept cookies, and I don't see anything else that should prevent me from logging on. I'm sure it's something simple, any suggestions?