And they do follow their own rules.

NW and CO will give you the instant upgrade on a Y fare if any seat is available in FC. (I learned something today.) Delta will give you the instant upgrade on a Y fare only if upgrade inventory (V or X) is available. At times the inventory seems very generous, and at other time it seems downright stingy as appears to be the case you are looking at.

There are a couple of ways to check for upgrade availability as described in an earlier post.

Obviously NW and CO have more favorable upgrade policies for this type of fare.

But this has been the way Delta has done upgrades for quite a while, at least for the past 4 years I have been a medallion.

https://www.delta.com/skymiles/about...ades/index.jsp

Complimentary Upgrade Terms & Conditions

Availability:
Unlimited complimentary upgrades are subject to availability, may not be available on all flights or in all markets, and are not valid for travel to/from Hawaii, Transatlantic and Transpacific destinations. Complimentary upgrades are not available on flights to/from South American cities that are serviced with our BusinessElite® product (i.e., Buenos Aires, Argentina; Santiago, Chile; Rio de Janeiro and Sao Paulo, Brazil). For travel booked using a SkyMiles Companion Certificate, neither the paid primary passenger nor the companion passenger will be eligible complimentary upgrades. We limit the seats available for use with complimentary upgrades and may not make seats available on all flights.

David