There are a few reasons that you may not be able to add a particular product to a kit:
1) If a product has a pricing calculator assigned as the pricing engine, then it won't be available in the kitting items dropdown. Only products using a UOM or price break pricing engine are available to be used in kits.
2) If a product uses an eDocBuilder template, then it won't be available in the kitting items dropdown. Only static items can be used in a kit.
Please refer to the chapter for more information on Products (Kitting)