LEFT JOIN shouldn't it work like this...?

I'm using Access 2007.

I have 3 tables.  The first is Invoices the second is Payments and the
third 
is PaymentAssignments (I’ll call it “PA” – it’s an intersection
table between 
the Invoices table and the Payments table).

I want my query to combine fields from both Invoices and PA and to return 
ALL records from Invoices where the fields from PA are null (they are null

only if there is no matching record).  (But I’m not trying to only get 
Invoice records with no matching PA records – I want ALL Invoice records

regardless of whether there are any matching PA records).

So I LEFT JOIN Invoices to PA and add criteria “WHERE PA.Field1 is
null.”
But the problem is then I ONLY  get Invoice records that have NO matching 
record in PA.

With NO where clause, shouldn’t I get BOTH an Invoice record where the
field 
from PA is null AND all Invoice records where the field from PA is not
null?  
I’m only getting one Invoice record (the one where the field from PA is
not 
null).  
Take Invoice #100, for example.  If Invoice #100 has a matching record in
PA 
then my query with NO WHERE CLAUSE returns one record for Invoice #100.  
Shouldn’t it return 2 records for Invoice #100 (one where PA.field1 is
null 
and one where PA.field1 is not null)?

What am I missing?  Your help is appreciated, thanks!