I've had a recent situation where BA told me that they would be sending me some Avios - which didn't arrive.

Fortunately the agent called me back a week later about a non-directly related issue and I was able to thank her for deciding to send me the Avios, but that they hadn't arrived.

Apparently when this happens, the agent should get a rejection notification but this wasn't working, so she would only have found out if there hadn't been follow-up. She was then able to re-send without an issue. So it's possible that there are more people who have been expecting Avios and they won't ever arrive without followup.