Why certain line items are not added to bill when converting multiple purchase orders to bill from purchase order list page?

This happens when the missing line items are a part of a Purchase Receives. In such case, you can to create the bill from the purchase receives. To convert a receive into a bill:

• Go to the Purchase Orders module under Purchases from the left sidebar.
• Select the preferred purchase order.
• In the purchase order’s details page, go to the Receives section.
• Click Convert to Bill on the right corner of a receive.
• A new page will open along with all the details automatically fetched from the receives.
• Enter the necessary details and click Save.

Your purchase receives is now converted into a bill.