I had been machining standard parts for a customer when the 3x8 inch parts that had previously held down to the spoil board would not hold down to the spoil board. So for those particular parts run I fiddled with the tool path until I could get them to hold down long enough to machine completely.
In the normal course of using a spoil board the surface is machined on both sides to remove the layer of wax that blocks airflow through the board which is necessary for the vacuum to hold parts down. To remove the surface defects left behind when you cut parts completely through releasing them from the scrap material after each run of parts the surface of the spoil board machined once again. This process is repeated until approximately a quarter of the spoil board is used up. The spoil board is then flipped and the process repeated.
I only discovered the real reason for the sudden lack of hold power on the 3x8 inch parts when I flipped the spoil board over to begin machining from the second side. In my attempt at trying to extend the life of the spoil board on earlier parts runs I had discovered that I could get by with machining only one side of the waxed surfaces. This worked fine on parts with more surface area and did work fine until I had the 3x8 inch parts to machine.
Lessons learned are sometimes when you make changes to your processes you miss a potential pitfall. At times no matter how much problem solving you do the answer comes as an unexpected surprise.