There is a little quirk with NULLs when using the NOT IN qualifier. I use the term "quirk" loosely here because the behavior is exactly as intended, though it may not be obvious. The following query shows a trivial example:
At first glance, one might think that we'll see returned the row for the Chevy MINIVAN. However, when you run the above query (with the default option of ANSI nulls set to ON), you'll always receive zero rows returned. Why? Because the NOT IN is still a set-based comparison, and with ANSI_NULLS ON, comparing any value to NULL yields False (technically it yields NULL, but you see what I mean).
A quick solution can be found, of course, by simply adding a NOT NULL to the lookup column in your subquery. This isn't rocket science, and really isn't an advanced T-SQL topic, but it's one of those thing that can sneak up on you if you're not expecting it, particularly if you have one of these deeply nested in a complex query.