BariatricNurseD said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

There is a second half to this that has not been said yet. Start from the measurement rather than the conclusion. Almost every disagreement here turns out to be two people measuring different things and comparing the numbers anyway.

Closing the loop on my own question.

Follow-up: it was dead space. Switching to a low-dead-space fixed-needle syringe gave me the tenth dose back.

One thing that is still open after BariatricNurseD’s answer:

How long did you give it before you decided it was working?

LarryQC_SD said:

Switching to a low-dead-space fixed-needle syringe gave me the tenth dose back.

Careful with the causal language. Two things moving together in a period when five things changed is not a mechanism, and stating it as one makes the thread less useful to somebody reading it later.

LarryQC_SD said:

Switching to a low-dead-space fixed-needle syringe gave me the tenth dose back.

Right, and the exception is worth naming rather than left implied, because the exception is where people get into trouble.