Two motors are rotating slow in gazebo simulation

Hii, i am quite new to px4, in px4 repository i changed the iris.sdf quadcopter to my own made hexacopter, but the problem is when i give the arm command, i see two of the motors rotate slow compared to other 4 motors, please someone can help me out? Also is there any way to load this sdf by default as an hexacopter instead of changing the frame type from QGround control everytime?