What you don't give are the vital clues needed to help you; how you are putting the images onto USB, what software you used to do it and what operating system you are using, whether or not you formatted the drive, etc. etc. Nevertheless, I'll make some assumptions...
I assume you have formatted the flash drive as FAT32. Format it at as FAT16, not FAT32, then try again using Unetbootin to put the image on the flash drive.
<aside, for those who don't know>
Yes, I know FAT32 is the default, regular format to use. However flash drives that fail to boot and are formatted FAT32 will often work flawlessly if the drive is formatted FAT16. Go figure.
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