[Veselin Kolev]

What happens when you shift when the motor speed is really low? From what you posted, I'm taking that the dog is sticky on the hex shaft and/or the servo isnt strong enough to shift. As the servos generally ARE strong enough to shift, you should probably take apart the dog/pin/hex assembly and file the corners of the dog and the slot for the pin that goes through the dog.

From my experience with dog shifters (Ive made 3 different designs), shift resistance has ALWAYS been due to friction in the dog assembly, never had anything to do with the gear speed, torque, etc. Just take the thing apart and file it and it should be fine. The servo should be plenty strong.