RD-6400 can't shift into low gear with large chainring
I recently purchased a new old bike off of eBay. Drivetrain is all Shimano 600/Ultegra in what appears to be good used condition (at least not obviously worn to my untrained eye).
The RD needed adjustment though and although it now shifts quickly and accurately with the chain on the smaller (42 tooth) chainring, I can't shift into low gear with the chain on the outer (52 tooth) chainring. The chain just kind of sits up on top of the sprocket without meshing into it. I know this gear combination isn't recommended, but it still should work, right?
Similarly, I can't shift from the inner to outer chainring if I'm already in either of the two lowest gears.
Chain is lubricated and not excessively worn (< 0.75% per chain wear indicator).
Could the chain just be too short?