I just did a check JFK-SAN and the low level 25,000 mile award showed Upgrade Eligible but the 32,000 mile level Not Upgrade Eligible I don't think it's low-high level deal.

Clicked on that hyperlink and copied this from the window that opened:

Not Upgrade Eligible
• This flight is not eligible for complimentary Medallion upgrades. This could be caused due to several reasons including but not limited to the following:
• This flight is already in the first or business cabin.
• The type of aircraft does not have a first or business cabin
• The class of service is not eligible for complimentary Medallion upgrades
• The flight segment is not eligible for complimentary Medallion upgrades
• The customer does not meet certain terms & conditions to be eligible for a complimentary Medallion upgrade

Perhaps it's already full?