I'm pretty sure that checkbox is not a "print" option, it is an indication of whether there is an Extended description for that line (which can be edited in the data entry screen using the button above, overriding the item's default description).  Someone probably edited the line description to be less than 30 characters, so there wasn't a need for extended characters.

The check box field in the line item is a read only. It is checked by the system if the description is longer than 30 characters. You can look at the full description by clicking on the icon that Kevin is showing you.  What MIGHT be happening is that someone deleted part the description until it was less than 30 characters.  Note that the description field is specific to the sales order.  I would ask around to see if anyone is editing the description.

When printing there is a check box on the printing panel to print the extended description. On the Standard form you cannot set this as default. You have to create a custom form and save which saves the settings. If someone unchecks the box it says unchecked until someone checks the box again.