When a campaign is set to "active", by default the emails/review requests will only be sent out to new orders that come in from the time of activation.
Amazon Campaign:
If you want to have the Amazon campaign send to previous orders, you would first set up the campaign and when saving select to apply to past orders as seen in the screenshot below.
Custom Email Campaign:
If you want to have the custom email campaign send out emails to your past orders, make sure you are targeting "Delivered" orders and when you click "review and save" in your campaign editor, select from dropdown box to send emails to past orders for the custom email campaign:
You can select up to 30 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.