What is the term for the M9 service pistol failing to eject the cartridge?

The term that accurately describes the M9 service pistol's inability to eject a cartridge is "Failure to Eject." This occurs when the spent casing does not leave the chamber after the round is fired, which can lead to a situation where the next round fails to feed correctly, causing a malfunction in the cycle of operation.

In contrast, "Feed Malfunction" refers to issues related to a round not being fed into the chamber properly from the magazine, while "Magazine Not Seated" indicates that the magazine has not been properly inserted into the pistol, preventing it from feeding rounds into the chamber altogether. "Stoppage" is a more general term that can encompass any failure to cycle the weapon, including a failure to eject, but does not specifically identify the ejection issue itself. In this context, the most precise term for the situation described in the question is "Failure to Eject."