Did they definitely follow your link and add their email to the list? I believe it’s the adding of email that bumps you up the queue, not the downloading of the app
Just to be honest… I noticed I got bumped unexpectedly several times and I only shared with one other person. So either that process is helped along or didn’t work right.
Judging by Monzo’s mentions on Twitter this issue is still happening regularly
If you send @Monzo a tweet or email help@monzo.com (include / use the email address that you singed up with, for the latter) they should be able to resolve this for you though.