I enabled Alternate Bidding, but my supplier cannot select any items for alternate bids. Why?

I set the following:

• Event is an auction with Parallel timing, or a competitive Request for Proposal (RFP)
• Rule, "Can participants create alternative responses?" is set to Yes.
• Desired terms in line items are editable in alternatives, set to Editable by Owner and Participant.

However, when opening the Alternate Bid window, my supplier cannot select any of the items, and the check boxes are all grayed out.

The supplier has not yet submitted an initial bid.

Suppliers must submit their initial bid for a given item before they can submit alternate bids for that item. After submission of an initial bid, the supplier sees a confirmation message at the top of the page, as well as the Revise/Alternative Response:

On the following page, after selecting Alternate, the items already bid on will be available for alternate bidding:

Strategic Sourcing