Are your line items in a portal - a related table? Sounds like it is either a relationship problem OR it could be that the portal fields are overlapping the portal boundaries.
What is the relationship you are using for the order form and line items?
I've done a similar setup in the quoting system we use in our database.
For me it's easier to show than explain so i created a sample file and have attached to this post. Wasnt sure what version of FM you are using but the file is version 12.
From the Orders layout, there is a portal of line items. clicking in the first field will show a list of products. Once you select a product, the information for the product is populated into the other fields.
Hope this helps you achieve your end result.
LineItem.fmp12.zip 11.1 K