Subforms Properly Related?

[Jaybird]

I have a main form (frmInvoice) based on a a table (Order Entry).

On it, I have several subforms related to Invoices:
sbfInvoice
sbfHDRPLAT
sbfLINPLAT
sbfContacts

The recordset of frmInvoice is restricted to those Order Numbers that are
associated with an Invoice Number (INVNUM) by a query. This is to avoid an
error when moving to records that are not associated with an invoice.

frmInvoice is linked to sbfContacts by CustNo (I have no problems with this
subform)
frmInvoice is linked to sbfHDRPLAT by Order Number(PK)/JOBNO(PK) - this is a
one to one relationship.
frmInvoice is linked to sbfInvoice by Order Number where Order Entry.Order
Number is a primary key. The relationship is one to many.
sbfHDRPLAT is linked to sbfLINPLAT by INVNUM (sbfLINPLAT is embedded within
sbfHDRPLAT) HDRPLAT.INVNUM is a primary key and LINPLAT.INVNUM is part of a
multi-field primary key. The relationship is one to many.

All of the fields on the form and to their underlying tables are updatable,
which would seem to indicate that the relationships between them are correct,
but if I move to a new record, I get the error that an index cannot contain a
null value. This says to me that they aren't related properly after all.
I'm confused.

Can anybody tell me from any of this what the heck is going on?