A board held in a relatively stable position, such as on both sides, will require less force being applied by the breaker to break the board, than when the board is held suspended by one side. This is because of the negative force exerted on the board by holders. When the board is held on two sides, this force is added to the force behind the punch which effectively doubles the force being used to break the board. When the board is held on only one side, there is little negative force being applied, so the breaker has to provide the full force required to break the board. If you are uncomfortable with the ability of the holders to hold the boards tight, you will have to compensate by providing the all the necessary force yourself, rather than depending on the reverse force provided by the holders.
Board holders are the responsibility of the breaker, if they are not performing adequately, get new holders.
Check the board holders for: