I'm not quite sure whether I have understood the question correctly..
You appear to be asking why no customer number is displayed when you start a new record - but I think my response is - why are you expecting it to be there? Once you start a new record then you have to supply ALL the information unless you have (VBA-) coded it to auto-fill certain controls. How would Access know that you wanted a new record for the previous customer rather than for a different customer unless you program it to do so.
If you want to repeat the last customer number then you have to add code to the form current event and create a variable to store the customer number.
SO at the top of the form module
Dim LastCustomer as long
In the form current event....
if me.newrecord then
me.customerid= lastcustomer
else
lastcustomer = me.customerid
end if