Disk manufacturers like to use one definition of kilo/mega/giga/tera-bytes while the operating systems like to use another definition. This results in actual capacity reported by the operating system being lower than what people expect based on manufacturer's specifications.
32,000,000,000 bytes would be called 32 Gigabytes by a manufacturer (using powers of 10) or 32,000 Megabytes or 32,000,000 Kilobytes.
The same capacity using powers of 2 (2^10 = 1024) as used by operating system would be:
29.8 GB ~ 30,517MB ~ 31,250,000 KB.
Your "32GB" drive would have started as a 29.8GB drive as reported by the operating system, and you most likely haven't lost any capacity.
There can be hidden files taking up space on the USB drive, however, and you might also have a recycle bin that isn't empty.