When a campaign is set "active", by default, emails will only be sent out to new orders that come in from the time of activation.
If you want to have the campaign send out emails to your past orders, when you click "review and save" in your campaign editor, select from dropdown box to send emails to past orders.
You can select up to 60 past days, but be aware there is a daily email sending limit imposed by Amazon (5X your daily average sales + 600 emails). We recommend to only target no more than 7 past days at a time. You can repeat this step each day and not worry about sending duplicate emails as within each campaign it will know not to re-send same email to orders that already have emails sent.
If you did do the past days option already, the reason why emails don't queue up is because this campaign is using the "shipped" status. For most of your past orders, they are no longer in "shipped" status, they are "delivered" status, so the system will not queue up these orders.
The second option to send to past orders is to use the order manager to send emails. Click here to learn how. Keep in mind that customers can only leave feedback within 90 days from their purchase, but customers can leave a product review anytime after a purchase.