Why won't my Mac Book Pro recognize my Flash Drive?

So I have never experienced this problem before. My flash drives always appeared on my desktop, then one afternoon, my usual flash drive was not showing up on my MacBook, so I assumed it was the flash drive and not my computer. Since that day, I haven't really used a flash drive for anything until today when I tried to save a few photographs on a different flash drive this time. I plugged in the flash drive in both usb ports and still nothing. I cannot find my flash drive anywhere! I checked all my system preferences and everything seems to be right so why am I still having this issue? I don't have a problem when i plug in my ipod or ipad, and I can't find my flash drive in Disk Utilities either.