Could they be hidden in a cave system? I've had some where I had to break rocks to get into a tunnel, or where the entrance was small and overgrown or hidden behind a waterfall. Gliding around at a distance can help sometimes, as sometimes the nodes show up through other scenery when you're at the edge of object draw distance.

If you're really stuck, press F1 to open console commands, and type 'killnodes'. That will remove all nodes on the island so you can carry on. Good luck! :)

I've got a fun one that even the command doesn't work on