Cant find Zorin OS in GRUB menu

Hello everyone, I have a problem. I today installed Zorin OS Core 16 with Ubuntu as main system. Everything has going well, but after I restarted laptop, I saw GRUB menu, but I didnt saw Zorin OS. In GRUB menu I found only button to go to Ubuntu and advanced ubuntu functions. I saw at other forums that that this problem happening at Zorin OS Core 16. I tried find tutorial at YouTube and other forums but they show about Windows & Zorin OS not Ubuntu & Zorin OS. I also posted about it at Android Forum. Please Help me.

Ubuntu System: Ubuntu 21.10 GNOME
Zorin OS System: Zorin OS 16.00 [dont remember]

Thanks!