Why do I keep getting “Boot error” when trying to boot unbuntu from a flashdrive?

I used unetbootin to make a bootable jump drive of a lubuntu iso image, but when I try to boot from it it just says Boot Error. I know it isn’t the iso because I’ve burned it to a cd and booted from the disc and it worked, but I’m going to be using the jump drive on a pc w/o a disc drive so I need to boot from a flashdrive. does anyone know what the problem may be? I’ve got the bootup command set to 1) removable, 2) hd, 3) disc (because I’m trying to boot from the flash drive) when i had it from the disc I changed it to 1) disc, 2) hd,… of course)