I would check these things:
If the email from all the apps are very similar and sent in a short period of time within each other, maybe it's being blocked by a spam filter.
Often, though, issues like this are caused by tiny mistakes that are hard to spot if you've been looking at it a lot.
The set up for Quickbase user and non-Quickbase user can be different. There is a setting on emails to enforce or ignore permissions that the user has for their role. Just some things I would check if I was having the same issue.
If you checked absolutely everything you can think of and cannot identify any possible cause, then next best step may be to contact Quickbase support who can look and see if there is a bounce event or otherwise verify if the email is being sent and delivered, and if not, why.
