Unlocking a parent item on mobile devices needs a kind of hack, described in https://www.joomla51.com/tutorials/item/mobile-menu-linking-parent-items
Unfortunately this doesn't work. At least not on an iPad 2017 or an iPhone SE with the newest iOS (11.4), template Revolve.
It only works by holding the parent item for a few seconds and then choose 'Open in new tab'.
But a visitor will not be supposed to know this workaround - this is an important failure.
So please find a new way to unlock parents items on mobile devices,