Default entry on main form

[giggles]

Im quite new to Access and have come across the following problem:

I have 1 main form with a sub form for showing PO (purchase order)
lines.

The form calls 2 tables, PO and POlines (1 to many relationship)

The main form calls the PO table which has a primary key of PONo which
is set up as an autonumber.

The main form has a 2 text boxes + 2 combo boxes. The values in all
the main form are all set to default, i.e. with date (text box), time,
Supplier (combo box).

However if I make no changes to the combo boxes or text boxes in the
main form, and go straight to entering the product code on the subform
it doesnt write the PONo to the POlines table. It shows the lines in
the POlines table but doesnt have a corresponding PONo.

In the PO table no details such as date and time appear.

HOWEVER, if i choose a different supplier by using the combo box, it
changes the auto number to the correct value and writes the PONo into
the POlines table and stores the PO details (date, time supplier) into
the PO table.

Can anyone help, or push me in the right direction. Im not sure which
topic i should be looking under to get more help

Thanks

Giggles