Problem in Unlocking Bootloader

Unable to unlock Bootloader by the normal ways? Know from our experts what alternatives you can try to unlock your Lenovo mobile Bootloader

I want to unlock the bootloader of my Lenovo A6600 Plus mobile.
But after entering into fastboot mode & connecting my device with PC via USB cable, whenever I type the command 'fastboot oem unlock' in command prompt, there are two options showing in my mobile:
1. Press volume up button to unlock the bootloader
2. Press volume down to cancel
But whenever I press volume up button, it's not working. Only the volume down button works which cancels the command & it won't let me unlock the bootloader.
Please suggest me what to do & how to unlock the bootloader. My volume buttons are working properly but in this case volume up button not works.
Please solve.