Hey,
Make sure you are using the latest version, secondary group support was added there. If that is already the case this could be the reason why you didn't receive it:
If you are the author of the post, you won't receive a notification (since you made the post yourself, you already know about it, so there is no need to send a notification to yourself)
Ie: if you as admin mention @coaches in your post and you are also part of the coaches group, you won't receive the notification (you are author) but all the others who have coaches as primary or secondary will.