Some expenses are not displaying on a vendor bill.
One reason for this could be that these expenses are flagged as reimbursable. The 'Reimbursable' flag is used only in CORE in context of employees when they charge reimbursable expenses against projects and have to be reimbursed by the company (say for mileage or travel expenses). In case of vendors, it is understood that they are already reimbursable and the company needs to pay the vendor bills.
This could also happen if you created the vendor bill using the Accounts tab. Vendor bills aren't required to have a project associated to them here, and thus expense entries never get made for them.