Design Manager is not accessible (access denied) to few designer in the Subsite even though they are having full control as an owner.

Design Manager is not accessible (access denied)  to few designer in the Subsite even though they are having full control as an owner to the site.

They have been given full control from the root site collection as well as in the subsite.
Still they are getting access denied message while trying to access it.
Even granting access through the access request sent by users is also not resolving the problem.

Reply at the earliest as it is quite urgent.

Thanks