I am trying to book a RTW ticket on *A.

The Lufthansa Book and Fly site and call center have availability on all the flights I want, in D class, but they can't use a credit that I have on UA from a cancelled ticket.

The UA Star Alliance desk can't see D class on two SQ flights that I need. On the GDS they show Z9C9J9D9. LH tell me they see D4 at least. UA see D0.

I was surprised by this, because I thought D class revenue was D class revenue, and the issuing agency would make no difference.
Two agents (HUCA) told me that nothing could be done, they had seen this many times.

UA also could not find a way to make my flight credit ($10K) available for LH to use.
LH could not give a PNR for the reservation which UA could book.
Both said there was no-one at SQ or above them in their hierarchy who could help.

On my 4th call to UA I got into Y on one flight but it won't clear automatically if D becomes available.

Any suggestions for a way to pursue this? I have zero flexibility on flight times unfortunately.
thanks

Since the merger, on many flights, 016 (UA ticket stock) D availability has been less than expertflyer D availability (particularly on SQ). If you do not care about PDQ's, book 220 (LH ticket stock) or another *A carrier.

It could be due to how UA agents are looking for availability.

Availability can vary by point of sale country, and also vary between O&D and married segments.

Actually there is a difference of over $400 on the CRTW2 fare for identical flights, but since You had no availability on SQ I had no choice. (the difference is in addition to the surcharge that SQ charge)