Another strange MMM observation in behaviour

so now the V3 is in and up and running like a champ. i fitted my hitech 645 mg servo for steering, however when i went to put the other 645mg for the throttle/braking the unit wouldnt initialize/arm at all. i reversed the channel and no luck either way. i put the original traxxas one back in, which i assume is a generic hi-tec maybe and it works like normal. i would understand if the steering servos didnt work, but they do. so i am not sure whats up with the throttle servo... any ideas? i even lengthened the arming time to 2 and 2.5 seconds also. no luck. for now the generic traxxas one will stay in there until i figure it out.