I can't find NixOS's grub entry

I’m trying to install NixOS from Ubuntu. But when I reboot I always boot Ubuntu directly and there is no entry about NixOS. What am I missing? There are three dirs in /boot/EFI: ubuntu, BOOT and NixOS-boot.

    boot.loader.systemd-boot.enable = true;
    boot.loader.efi.canTouchEfiVariables = true;
    boot.loader.grub = {
    enable = true;
    device = "nodev";
    efiSupport = true;
    useOSProber = true;
    extraEntries = ''
        menuentry "Ubuntu" {
            search --set=ubuntu --fs-uuid 45508ab8-2542-47c5-8159-0d9e4ef9e59c
            configfile "($ubuntu)/boot/grub/grub.cfg"


Have you checked your EFI boot menu if it has another set of choices?