Custom Code in Template: If you're using a custom template (which might include custom code like Jinja templates), check if the code is correctly referring to the product data. For example, check if the product data is being correctly passed to the template or if there are any errors in the templating logic.

• If you're using Odoo's QWeb templates, ensure that the product loop (for product in order.lines) and corresponding field accesses (product.name, product.price, etc.) are correct.
• If the field names or object references are incorrect in the template, the products will not display.

Or 

Missing Fields

Field Mapping: Ensure that all required product fields (such as product name, quantity, and price) are properly mapped in the receipt template. If any required field is missing or incorrectly referenced, the product might not appear on the receipt.

Custom Fields: If you're using custom fields for products or orders, ensure those custom fields are correctly defined and available in the template.

and check Error log 

Check for Errors: Inspect the Odoo logs or the browser console (for web-based POS) to see if there are any errors related to the receipt generation. Errors might give you clues about missing fields or broken logic in your template.

Server Logs: You may also check the Odoo server logs for any potential errors related to product scanning or receipt generation.