Hello Valentyn 
My guess is they don’t receive the digest depending on either the digest settings (which require a user to be away from the forum for a while in order to receive one), or the personal settings of users (they can chose to opt-out from digests).
At first, I’d compare the digest settings, the last time the user was seen, and their digest email preference (Activity Summary here: https://meta.discourse.org/my/preferences/emails).
As for the push notifications, I can’t answer as I don’t know about them, I never use it.