Hi,
I had two situations with booking partner flights via United.com or app. Hope FT experts can share some insights:

I am having hard time booking NH flight from HND to SFO (direct or via LAX) on Premium Economy. For HND-SFO or HND-LAX-SFO, Untied assigned A class on NH and gave me the price. After I clicked "Buy Now", I got error message to call United. Web Support agent told me that she couldn't plate the flight to A class even she saw the A-class inventory. She said NH did not release the seat at lower price during the time of purchase. There is nothing UA can do.

From the screenshot from EF below, I can see the PE inventory of NH108/UA7939 is not matching (E0 vs A2), although the highest PE matches (G5 vs O5). Is this the reason why I couldn't purchase an A seat of UA7939?

Separately, I was booking FRA-SFO, also on PE. One segment was AC-operated FRA-YVR. After the ticket was issued, I saw a note "you are on waitlist" under this segment.

So it does not matter if the ticket is issued or not, with a United codeshare flight number and inventory, it is up to the partners to approve the purchase? And is it common that inventory does not agree for several days?