What's your advice for when people don't respond, or respond very erratically? I'm talking about family members who I love and get on with when we're together in the same room, but repeatedly fail to respond to my gentle messages.

This advice is like anything, it's context-specific and mostly aimed at privileged, well-adjusted people. If you've got struggling friends or family, 'always reach out' is probably shitty advice and won't make your life better. It might make it worse. People who are struggling in life need out-reach the most, but will likely respond in the worst way (or not respond at all). Good luck with 'always reach out'.

Sorry to be negative, I'm venting - this type of advice about avoiding life regrets always rubs me up the wrong way. Similar to completing a 'bucket list' - more advice for privileged people. Some of us are just trying to survive out here without being reminded of all the regrets we'll have, if we even make it to old age.