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I have a new bike with Shimano Ultegra di2 12sp and 11-34 cassette. Following Shimano's installation instruction, with largest sprocket and small chain ring, I cannot move the upper pulley close enough to the largest sprocket even with b-screw all the way out. Shimano suggests 6mm and mine is over 10mm.
Shifting seems fine so far, but I'm wondering if something is wrong.
Something seems unconventional with the angle of the connecting link from the rear derailleur to the frame attachment point. It appears to be pointing up from the frame going back to the rear derailleur, when it should be level or even pointing toward the ground. So something is amiss. Here are some pictures I have found that show the link pointing rearward but slightly down, instead of up as in your photos. This would definitely account for the lack of B-screw adjustment, and lack of chain wrap around the cassette cog as well.
This image below, the link is pointed down and back (CORRECT).
Another image with the correct link geometry, the link is pointed down and back as well.
Your supplied image, however, has the link pointed back, but also sharply up instead of down. This is what you should investigate first. Once this is resolved, your b-screw will likely be able to adjust to the correct gap of the upper pulley to the bottom of the cassette cog.
One additional "tell" is the location of the upper pulley's front-to-rear position relative to the rear axle. In the first two images, the upper pulley appears 1-2 inches forward of the rear axle. In your image, the upper pulley is slightly behind the rear axle. Granted, there will be some forward and rearward movement of the upper pulley depending on the gear (front and rear) being used, but the differences between the pictures appears excessive.
To resolve the issue, shift both the front and rear derailleurs to their smallest chainring and cog, respectively. Next, remove the rear wheel. This will take tension off the chain and will make the following steps much easier. Next loosen the b-screw if it is tightened in a bunch (already done) and follow that with loosening the rear derailleur attachment to the dropout (an inch below the rear axle). You can probably do this without removing the chain, DI2 wiring, and possibly you won't even have to remove the rear derailleur completely (you can if you wish and it would provide some visibility of how the mount should fasten into the correct positioning). Once you have realigned the link to the proper orientation, tighten it down. Then go back to adjusting your b-screw. You may need to let the RD re-index itself, or maybe not, but be prepared for it.
Here is an image of how the angles should look, directly from a Shimano web page. The bolt circled in red is the one you need to loosen to adjust the alignment of the link. Shimano Link:
A chain that is too short could also display a similar tendency, and that should be investigated as well, but after the rear derailleur geometry issue is resolved.
