Usually this will be because they haven't opted in to receive promos from you. They will get an opt-in link when you first send them a promo and if they don't opt in they won't receive anything until they do. You can tell by the red/green dot next to a contact whether or not they are opted in.
Best thing is to ask anyone who is having issues receiving them to check their spam etc for that opt in email, and/or you can also generate a new one for them by selecting the 3 little dots next to their contact and "copy opt in link", and then send them that manually.
Alternatively it could be due to your Privacy Policy being updated at some point, if you selected 'Resend to Contacts?' = Yes - this resets the opt in status of all contacts and they will need to re-opt in order to receive your emails.
If you have anyone who has opted in and is still saying they are not receiving them then please reach out to our support team with their name/email and we can take a deeper look into it.