Hi,
We noticed we quoted you last week on 3 boxes but we did not receive an order from yourselves.
Could you let us know where we went wrong?
We quoted you last week on 3 boxes, but we did not receive your order.
Could you let us know where we went wrong?
A week is not long to wait. Perhaps they just haven't had time? 
Anyway, I think you need to add more politeness and also say how much you want them as a valued customer.
Clive