There are several possible reasons for this situation:

1) You have a stuck order. For example, you accepted an order that requires some cants, but your storage contains only rough lumbers. You cannot fulfill the order in this situation, and it gets stuck. Cancel the order and get a new one for the materials that you have in your storage.

2) Your storage is full of products. Try to fulfill orders more frequently. You can also activate Samuel's recurring order (he will fulfill orders automatically) or run Flash Sales.