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 Purchase Orders under Procurement on the left sidebar.
• Select the preferred purchase order.
• On the purchase order’s Details page, go to the Receives section.
• Click the required receive.
• Click Convert to Bill next to the 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.