Check that your product has a valid Product Sync under Data -> Product Sync (you must be connected to your accounting platform and have the correct permissions.
2. If you have enabled Orders Deplete Warehouse Inventory (Warehouse Management) in your Account Settings:
Make sure that your products have been individually "Enabled" for Warehouse Management. A large bar at the top of the Manage Products modal window will inform you of the status of each product. Click the large bar to Enable/Disable Warehouse Management.
3. If you are using Product Groups to limit the available products to individual Direct customers:
Make sure to add any new products to existing groups to allow them to appear on order forms. Customers can be assigned multiple Groups. If a new Group is required, be certain to add the new Group to Customers where you require those products to be available.
4. Make sure you are using unique SKUs
Duplicate SKUs can occur on bulk upload. If two products have the same SKU, only a single item will appear.
5. Make sure the product is Active AND Available
All products can be set to be Active and Available. Both need to be true for the product to show up on an order form. If Active is false, it removed the product from everything - including reporting (designed for archived products). Available set to false would indicate that you are unable to sell this item (either currently sold out, it's a seasonal, etc), but you still want the product available for reporting.