Selecting a product option from the drop-down menu does not update the SKU on the website

Selecting a product option from the drop-down menu does not update the SKU on the website.  I have confirmed the part numbers are correct in the attribute combination table, but I’m not sure where else to look.  What is preventing the SKU from changing to agree with the selected option on the website? Are there instructions you can point me to, possibly I haven't set this up correctly.

I am confused with what you trying to do ?
What is the product option from the drop-down menu ?
Which SKU is not changing ?
Please give more details or provide an image

I cannot reproduce the issue. It works fine. Just ensure that "Inventory method" is set to "Track inventory by product attributes".