You shouldn't be using Unetbootin anymore since years ago.

This piece of software comes from a time where .iso images of Linux distros were made solely for CDs and didn't run on other supports. Unetbootin did replace the bootloader on these images with one that would be compatible with USB drives and such.

Now, ISOs are compatible with nearly any storage type, and the Unetbootin replacement tend to break stuff on the distro installers.
Alternatives ?
Windows : Rufus
Linux : Google "dd iso usb"

I'll elaborate more on the dd part. I couldn't because I ran out of characters.

This is how you use dd to flash an iso :

dd if=/path/to/your.iso of=/dev/sdX
where sdX is your flash drive. Do not specify a partition number.

This erases stuff on the drive, so be careful about that. Also, to reuse the drive afterwards, you gotta create a new partition table and a partition on it for it to work.