UPDATE: The plot thickens. I measured 2.5V at the door even with the wires completely disconnected from the transformer. So, the transformer is contributing to the voltage difference but not solely. What an adventure!
I'm going to call an electrician and will update this post when I learn more.
I have a doorbell transformer in my basement, about ~30 feet from my front door. The wiring in the walls may be longer than that, but I'd be surprised if it were significantly longer. I don't know much about the wiring. There's no chime box and there's only one doorbell in the house, but it's possible that there may have been other configurations in the past.
I attempted to install a Nest Hello video doorbell today and failed because I couldn't get the voltage at the door to measure more than ~12V using my multimeter.
Initially, I had a 16V 10VA transformer. At the transformer, I was able to measure ~19V and at the door, ~11V. I bought a new, 24V 20VA transformer. At the transformer, I measured ~29V and at the door, ~12V.
I tried restripping the wires on both ends for cleaner connections to minimal benefit, and now I'm out of ideas. I'm also puzzled by why the transformers have a slight difference in voltage at the door but not proportional to their difference in voltage stepdown.
Any ideas? I'd love to be able to fix this myself (or hire someone if that's what it takes), but I'm not going to open up the walls.