I think you need 2 relationships for LineItems and Products
First for making value list, now you have this.
Second for getting description (or any information from Products table, for each line item)
You can make it duplicating Products table in "Manage database" Relationships tab (not Tables tab), then use it in "the normal way" relation.
Using a cartesian relationship to Products will enable you to make a list of ALL products.
Maybe the value list is defined as "include only related values starting from ..." ?
If this is the case, toggle that to "Include all values" and you're set.