Ubuntu not showing in grub menu after dual booting with Arch

Question

Earlier my laptop had only Ubuntu on /dev/sda5 and it's swap memory on /dev/sda6 in which I logged in through grub menu. I installed Arch on /dev/sda1 and it's swap memory on /dev/sda2 and again installed grub menu. Now when booting my laptop, I only see Arch Linux in grub menu. What should I do so that Ubuntu also shows in grub menu? I tried making my /dev/sda5 as "Bootable" in cfdisk but that didn't help.

Output of lsblk:

NAME        MAJ:MIN     RM   SIZE    RO TYPE     MOUNTPOINT
sda          8:0         0   238.5G  0  disk
├─sda1       8:1         0   194.5M  0  part     /
├─sda2       8:2         0       4M  0  part    [SWAP]
├─sda4       8:4         0       1K  0  part
├─sda5       8:5         0    36.2G  0  part
`─sda6       8:6         0     3.8G  0  part

Commands executed while installation for grub:

grub-install /dev/sda
grub-mkconfig -o /boot/grub/grub.cfg

Answer 1

Install os-prober then mount Ubuntu partition:

Detecting other operating systems

To have grub-mkconfig search for other installed systems and automatically add them to the menu, install the os-prober package and mount the partitions that contain the other systems. Then re-run grub-mkconfig.

Or add Ubuntu the /etc/grub.d/40_custom :

Dual-booting

Assuming that the other distribution is on partition sda2:

menuentry "Other Linux" {
    set root=(hd0,2) #  In your case : set root=(hd0,5)
    linux /boot/vmlinuz # (add other options here as required)
    initrd /boot/initrd.img # (if the other kernel uses/needs one)
}