Some times the answer is a surprise
I had been machining standard parts for a customer when the 3x8 inch parts that had previously vacuumed down to the spoil board would not hold. For that particular parts run, I fiddled with the tool path until I could get them to hold down long enough to machine completely. In the ordinary course of using a spoil board, the surface is machined on both sides to remove the layer of surface wax. The wax blocks airflow through the board's surfaces and is necessary to generate the vacuum for holding parts down. 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 pieces 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 a surprise.