no update possible from 11 to 15.2

Hi, sorry for mi bad english, i have installed the firmware ver. 11 in my NS 2, all is work good, but wen i try to upgrade in 15.2 in web interface i have this problem:
SPD8020 firmware upgrade
step 1: uploading firmware package, successful completed
step 2: writing image to disk partition, successful completed
step 3: perform reboot
But, nothing appends, after reboot i still have the ver. 11.
Any idea?
Thanks
SPD8020 firmware upgrade
step 1: uploading firmware package, successful completed
step 2: writing image to disk partition, successful completed
step 3: perform reboot
But, nothing appends, after reboot i still have the ver. 11.
Any idea?
Thanks