Formula column expression editor - what determines whether a table shows up?

[RADSr]

I'm trying to build a formula column.   Using the lookup() function, but I don't suspect that's meaningful to this question.  

 - The schema has 45 tables in it.   Only 9 of them show up in the expression editor's dialog box ( left side ) 

- Of the 45 tables, 31 tables are SQL Database tables and come from the same source 

- I do have two groups defined for load order w/ 3 tables  in group 1  -- one of those tables shows up in the expression editor and the other two do not. 

 - Of course the ubiquitous law of Murphy's demands the table from which I want to lookup() is *not* in the list

- typing the table.column names nets me an error saying that I may be referencing a column which may have been deleted.

  --  suffix to above:  The expression *does* validate correctly and w/out issue.   

I welcome ideas and fully acknowledge I may just be missing the "show me what I want" checkbox in the UI.   ( but I don't think so!  😉    ) 

[RADSr]

Adding: I have the standard DATE_US schema and two other tables I've added ( date dims w/ additional attributes ). But when I try to add a lookup() function to DATE_US I only see the DATE_US table.

I'm sure there *is* a rhyme and a reason for Incorta's expression editor deciding which tables show up, but I'm flummoxed as to what it may be.

I wonder if we can start by determining if the lookup() function is supported when creating formula columns in the physical schema layer?