1st item to consider is email accounts temporarily go offline just like your isp.
2nd item is some people may still have a valid email account but don't use that specific email anymore.
3rd item is your host may be limiting how many emails can be sent per minute/hour/day/month.
So they throttle the number of messages sent ad do not allow all of them to be delivered.
You may want to contact your host about this item to verify.
4th item is the board email account may be blacklisted by some isp's
it is good practice to do it several times. but I would be using the inactive group for repeat emails.
So you don't nag your good accounts.
Then monitor future normal emails for bad accounts.
As far as the one member that never got one.
send one to him directly from the ubb
