Link Packing slips vs Load in a form
Adding data sources isn't enough; you must also set their properties accordingly. The Join Source property defines a parent data source of a given data source. The Link Type property defines the type of join (e.g. Inner Join) or link (such as Delayed). There should be a relation between those tables, if it doesn't exist and can't be added, you can add a link or a dynamic link in code.
Also, just checking whether you're getting correct data or not is not a sufficient way of debugging of a query. You should look at the actual query used by the form, because it'll give you details such as whether joins are in place, what join conditions are used and so on.