India, May 23 -- When Jeff Wells placed a reservation for a Rivian R1T pickup in early 2019, he was one of the first in line for a truck from the Amazon.com Inc-backed electric vehicle startup that at the time promised to tap in to a niche not served by other automakers.

But Wells, an accountant from Southern California, has become increasingly frustrated as he sees others, who placed their order years after him, receive trucks while he keeps waiting. "It's just annoying and it feels like there's no order to how they're doing things," he said of Rivian.

Wells is one of dozens of reservation holders who in recent weeks have complained about unreliable delivery timelines and delays in online groups and forums.

The complaints mounted afte...