Grub Customizer is an independent application made by another developer. I personally do not recommend it because it writes configurations that are stored in unexpected places - a user may be unable to revert the changes made by the customizer.
It wouldn't be offered on Zorin OS - I ask just in case it was something you sought out and installed.
Is windows Fast Startup or Fast Boot in Control Panel > Power Options disabled? It can interfere with other actions if it is enabled.
And - If Windows is not properly and fully shut down - it can prevent a user seeing grub or GnuLinux because you are not actually booting up the machine - but waking Windows out of Hibernation.