Rails ActiveJob/Sidekiq sending emails inconsistently; sometimes sends, sometimes doesn't

I have a Rails 4.2 app using ActiveJob/ActionMailer and Sidekiq/Redis with Devise for authentication. I require email confirmations when signing up, which is handled by Devise. I'm running into an issue when users sign up, but the confirmation email sometimes doesn't get sent.

The logs show that the confirmation email is enqueued by ActiveJob, and sometimes it gets processed, sometimes it does not. When the confirmation email isn't processed, and the user requests for it again, the new confirmation email goes through.

Any ideas as to why this is happening?

Also, I am not iterating over any users, I'm using the standard Devise sign up process. So this doesn't apply.