I'll jump in here as I've been diagnosing tranny problems after a rebuild.

What if anything has been changed over time? b2 piston? fluid/filter change.

Any adjustments to bowden cable and or vacuum modulator?

If you let off the pedal will it eventually shift or do you have to take it to high rpms to get there?

Does this occur on all gear upshifts? Could be the sticky elements in the valve body and/or worn K1 and K2 clutches and/or pistons not releasing the b1 and b2 brake bands which are associated which the Kx clutch packs.

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)