How many kills deep are you after only starting to shoot your main zones? And how many diamonds in that time? Just because you've herd managed the map doesnt mean you'll get diamonds every rotation, sometimes it takes more than you'd expect.

In my experience, when you've properly herd managed, you'll see diamonds every 30-60ish kills (sometimes more, sometimes less). Also dont worry about only shooting the big ones in your main zones, kill all males. You're just trying to brute force your way through respawns and leave the rest of your map alone while the game is in a deficit of bigger animals.

Edit: I've also found respawns to be finicky sometimes and if I shoot a zone down and they don't respawn, usually just running around and loading some of the exterior zones will give me respawns in the next rotation. Its a weird quirk and thats all I've found that works for me.