I assume youa re using a left join to make sure you get all the records.
If you use anykind of select statement on a field that come fom either table that has the fewer records it will change this to an inner join and the records will drop.
If you are just palcing a formula field on the canvas try altering to handle the NULL ID values to include them in the process.
If isnull(table.ID) then " " else formula here
Make sure you use the ID field from the table that has the fewer records.