an excerpt from the help section, read the second point as it might shed light on your problem

Bill of Material Overview

It is important to understand the differences between manufactured and kitted items:

• Good examples of manufactured items are glue, pizzas, and CDs. You take various components and manufacture it into a brand new substance. A good example of a kitted item is a computer. It contains a hard disk, memory, computer case, power supply, motherboard, and so on. One way to look at the difference is that you can return a kitted item into its components, whereas you cannot return a manufactured item back into its ingredients.
• Another major difference between manufactured items and kitted items is how you bring the inventory in. The only way to bring manufactured items into inventory is to use the Receive Inventory feature in the Inventory module. Once you do so, the item functions in the same way as a normal item from the sales point of view. For kitted items, the system can assemble the items automatically in the Order Entry module as well.
• You cannot use alternate items when assembling kitted items.