Why would a Select from Tabke1 where FIeldA not in (Select FieldA from Table2) fail if I manually confirm it should work?

Question

I ran

Select Term from TableA where Term not in (Select Name from TableB)

successfully. Yet when I tried another table

Select Term from TableA where Term not in (Select Name from TableC)

It returns a NULL set. I manuallyt confirmed there are literaly 1000s of records in TableA not in TableB and that the fields are correct.

Is there some other consideration I am not taking into account/understanding about what seems to be a simply query?

Update: Interestingly enough this alternative query worked:

 Select Term from TableA where not exist (Select Name from TableB where TableA.Term=TableC.Name)

So while I solved the problem at hand I am still curious why the first query worked for both tables but only the second worked on TableC