Solus does not use Grub assuming you are using EFI. Solus uses clr-boot-manager.
To update clr-boot-manager run sudo clr-boot-maanger update
Link: https://getsol.us/2017/03/26/clr-boot-manager-now-available-in-solus/
If you are using a BIOS based board then sudo grub-mkconfig -o /boot/grub/grub.cfg
"might" work.
I find clr-boot-manager nicer than grub for EFI systems. (Much simpler)
Let me know if this helped at all.