The only emails I get from stake is the ones I requested for each time I want to make a transaction ony account, so for that I can say that I fall into the same category of not getting any bonus notifications email from the stake team, and I think that those bonuses are for selected few gambler's may inactive ones I guess.
No need to think much about the bonuses because when the time come for you, the bonus will arrive in your account. You just gambling as usual and enjoy your time while they will give you the bonus when you deserve to get it.
They may give the bonus to random members and the selected members will get the bonus. But I am sure Stake will give more in the future to their inactive members to invite them join with the other gamblers.