In some cases, a template will show correctly but the products will be missing from the document.
This issue is unfortunately caused by a bug in the Shopify Order Printer app that has come up recently, which is caused by the "sorting" of the products in the template.
Basically, if it finds one of the products (a custom product) doesn't have the field filled out that is being sorted on (for example SKU) then it comes out completely blank.
The fix for this (until Shopify fix their issue) is to make sure all products have the field filled out that is being sorted on, or to disable sorting in the template temporarily. You can disable sorting in the template like this:
Go into Order Printer Templates (Shopify admin -> Apps)
Click the "Customize template" button for your design
Go into the "Options and formats" section of the editor
Change the "Sort products by" dropdown to "Original (unsorted)"
Press the "Export to Order Printer" button and then copy/paste over the new template code into the Shopify Order Printer app.