Troubleshooting Why a User Might Not Receive the Onboarding Message DM in Slack

Would there be a reason a user wouldnt recieve the onboarding message DM? I was opening up the DM window with a community member and noticed they didnt have the DM from me. Spot-checked a few recent folks and no issue with those, so trying to think why they didnt recieve it. does it not trigger if you invite members a specific way to the slack instance? i dont think so, but thinkiung out loud. 🤔