You either assembled it wrong or one of the high RPM gear wheels is loose (ie freewheeling) on it's shaft. (maybe the first one the center armature gear contacts).
Once the gear train assembly is properly pressed into the case past the 3 little holding divots, the center armature gear will NOT disengage from the gear train. It will if the gear train is not fully seated in the case or if a pivot has completely worn though the bearing plate backing disk which limits it's lateral movement. You also need to make sure the capillary shim does not touch the armature vane disks.
Posting some images would be helpful.