Some users not receiving email subscription

I have built and published a report with RLS (row level security) so users must subscribe to the report themselves.

I've made sure that all users have been given acces to both the report and also added to the RLS.

It is shared with a few hundred users and all are pro accounts. They are all able to see and use the subscribe feature but only a handful of users were able to receive emails.

What could be the reason for it not working for some? Those who do not receive emails also tend to have their subscription emptied when they go back to check what they have susbcribed to.