Thanks for reaching out varahala,
It sounds like your notifications are coming quieter than you'd expect. We can help.
First let's follow these steps to Adjust the volume on iPhone. Then, to turn up your notification volume go to Settings > Sounds & Haptics > Ringer and Alerts and drag the volume slider to the level you want it at.
If you have the Change with Buttons option enabled, you may also want to turn that off to lock the Ringer and Alerts volume. When Change With Buttons is enabled, anytime you adjust the volume for anything, the Ringer and Alerts volume will go to that level.
Turning off Change with Buttons will lock the Ringer and Alerts volume to the same level no matter what you adjust the volume to for other audio.
Hope that helps.
Take care.