Linux auto mount 's problem

MY linux is ubuntu . ubuntu mount usb automatly.

I visit my usb through command . I visit /media/myusb touch a file , and i try to chmod this file, and strange things happen, i run the command and everything is ok . but when i list file with ` ls -l ` looks not effect.

I umount myusb , and mount manually. rn `ls -l` and I see the effect.  by the way , I login as root.