I have begun putting the HTML code in my auctions so that the store window shows up instantly. It takes some time for all the auctions to be included in the window, but I listed some last night and they were all included this morning when I checked.
You may just want to give up, disable your showcase, and put the HTML code in yourself. I found it easy to do. I even added it to my active auctions last week and it worked just fine.
I agree that it's likely a glitch on Auctiva's part and that they will probably get it fixed, but in the meantime I believe the store window has increased the visibility of my items quite a bit. I'm getting bids early on now instead of having to wait for my items to "float to the top" on a search.
Hope you get yours working soon!
(you can see my auctions here if you want to check what I did)