Can't use USB as a boot drive

Basically, I'm trying to use my Transcend JetFlash 8GB USB as a live linux usb to switch between linux distros, but I can't seam to get either my laptop or PC to boot from it. I temporarily borrowed my sisters USB which seams to work just fine. My usb works fine i just can't get my PCs to boot from which I was able to do just two weeks earlier. Any suggestions?

Does it detect the USB?

I know this doesn't really answer your question but if you've got a rooted phone this is what I use instead of USB drives. https://play.google.com/store/apps/details?id=com.softwarebakery.drivedroid&hl=en

Finally got it to work again. I was using YUMI to try to set up a Multiboot drive but it wouldn't boot. Then I tried to use Universal USB installer form pendrivelinux.com and still noting, then unetbootin, then LinuxLive USB Creator, and it still wouldnt work. Than I used WinToFlash to see if it would boot up the Windows installer and it worked, after which i tried Universal USB installer again and now it works fine without any problems. Thanks for the replay's. Still, though, it would be nice to know what caused this whole mess in the first place.