It is possible that the initial email is ending up in user’s spam inbox. If you know the email address or username of anyone who is reporting the issue, you can check to see if the activation email has been sent to them from your Admin / Emails / Sent page. The Email Type for the activation email is ‘signup’:
The Post & SMTP Details section of that page gives details about whether or not the email was handed off to the user’s mail server. If you see 250 Ok in that column, the email has been handed off.
For the issue with the activate button not working, I wonder if users are running into the bug that’s described in this topic: Resend Activation Email - 404 Error?