Intermittent/quiet ring tone

iPhone 13 Mini. IOS 15.6.1

There are a couple of issues that I cannot resolve to do with alerting.

I seem to receive alerts (phone calls, Timers, SMS) intermittently. By this I mean that either I do not hear the alert sometimes or I do not receive the alert.

"Settings>Sounds & Haptics>Ringer & Alerts" is set to maximum volume. I have also experimented with "Settings>Sounds & Haptics>Change with Buttons" selected ON and OFF - no difference in outcome.

When I select a new tone (Ring tone, Text tone) the new tone is heard at what I expect is the correct volume.

Then two outcomes occur:

1. When I next receive a SMS/Alarm/Timer either there is no Alert or the volume of the Alert is much reduced
2. When I receive a phone call the first Alert is heard at a reduced volume and 2nd, 3rd etc repeats of the Alert steadily reduce in volume.

Any thought on this? Is there a combination of settings that could cause this?