system installation - Can't see bootable usb in boot menu

I am using Kubuntu 16.04 and I burned another linux distro to my usb via unetbootin and then I also tried dd comand in console. Problem is I can't see the usb drive in boot menu.. When I tried older usb in which I have burned kubuntu iso from the past i could see it. I also tried to burn iso to different usb but I still can't see it. I think it's a mount problem because it mounts only after I click on it in file manager. Any tips ?