USB stick won't stay mounted after reboot

[earlforeal]

I have had no trouble with this until suddenly today, after rebooting, the drive will not stay mounted. The only thing I can think of that might have caused this was reformatting the USB stick to FAT32 via Windows.

I use the command line to mount the stick,

$ mount /dev/sda1 /mnt/usb
$ lsblk //shows the stick is mounted
$ reboot
$ lsblk //shows the stick is not mounted

Any thoughts on where I went wrong?