Have you fully completed the pre-flight procedures on the FMS?  Do you have T/O indications in the ADI and EICAS?  Did you complete the EICAS section of the pre-flight checklist (set the computer selector to auto, pushed in the thrust reference selector)?

You said you have auto-throttle enabled.  Did you run the throttles up to 1.1 EPR before pushing the EPR switch?

Just guesses, really.  There are still a few things I can't figure out about the 757, but takeoff has never really been a problem for me.

Also, I think you still do have some control over the throttles.  Another user was explaining de-rated thrust to me and mentioned that advancing the throttles to their stops will override the thrust reference and go to max thrust.