like i said, i have no experience with unetbootin, only extlinux. if i find the answer i'll let you know, but it's not unlikely someone else will know and mention it.
i still recommend unetbootin because so many people here do, and though i prefer extlinux, i don't like suggesting it to anyone else- i don't like telling people to change the partition on their usb (or anything else.) if they've already changed it, that's when i recommend the method juanito and i use. posting the contents of syslinux.cfg may help.