[AccessD] Continuous subform on a continuous subform

Dear List:
 
I would like to put a continuous subform on a continuous subform, but I'm
getting a message that this can't be done.  So I'm trying to find a way
around this. 
 
The form tracks incoming shipments.  The main form is single form showing
the shipment info in the header.
 
Each shipment can have more than one purchase order. So the purchase orders
are listed in the detail section.  Form is continuous.  So far so good.
 
However, each purchase order can have several lots. And for each P.O. added
to the shipment the client wants to see all the lots attached to that PO.
The lots and purchase orders are already in the tables.  They're going to
create the shipment last (this incoming shipment thing is an add-on to the
existing legacy system).
 
So the user creates a new incoming shipment and adds existing purchase
orders (incoming shipment POs are in a table with the shipment table PK as
the FK) .  So I've got the purchase orders displaying in a continuous form.

 
But for each P.O. the client wants to see all the lots attached to that PO.
So I think, OK, subform of lots (lots have the PO number as a field) linking
the selected PO numbers to the lots.  Subform in a subform.
 
But no-go.  So, how does one finesse this?  
 
MTIA
 
Rocky