When copying folders the contents of their subfolders are missing

Are you sure the subfolders on the USB stick actually have data?

FYI, the quality of many USB sticks is extremely poor so it would not surprise me when issues like this occur. Try using another USB stick, or better yet a USB drive (hard drive or SSD) instead to see if it has the same issue. The SanDisk USB sticks tend to be a bit better quality than many others.

You may also want to check the source as well if the subfolders on the USB stick are empty. Maybe the issue is further upstream on the original drive/data.

Edit: Also, are you using any anti-virus apps, cleaning/optimizer apps, or third party security software?

What is the file system being used on the USB stick? Some file systems have file size limitations.

Palwebstar wrote:

That's the problem. e.g. copying files from a USB stick to the Mac, the folders copy with their contents, except that any subfolders are empty.

Copy how...?

drag & drop to move the data.

Hold the Option key before drag & drop to move a copy of the data

there are alternatives:'Manage files in Terminal on Mac - Apple Support