It seems it is partitioned into several drives and you can only see one of them, probably the boot partition.
If you are doing it in Windows, open the "Administration Tools/Computer Management", find the "Storage" section and it will show you the partitions on all your drives, including the USB one. It will probably say the largest part of the USB one is unrecognized (Windows can't read Linux partitions). MAKE SURE YOU SELECT THE CORRECT DRIVE then click on all the partitions in turn and delete them. When they are all gone, create a new partition using the whole area then format it. The stick should then be like new.