One of 2 things.....how are the flap servos mounted? Unlike an aileron servo...flaps need to be the same way In other words the servos need to face the same way or the servo arms need to be on the same side....ailerons are opposite of each other.
Or if you're controlling the flaps through 2 channels reverse the wrong channel
If you made a mistake installing the flap servo I bet you could use a reversing Y to correct the travel