Yeagarup Dunes

Last weekend I was down in Pemberton, followed the map provided by the local tourist centre pointing to Yeagarup Lake. To my amazement, there was no signage on the Vasse Hwy, noticed the map does show the turnoff was close to the track leading to Old Vasse Rd.
After arriving at the lake, had cuppa tea, then proceeded on the track to the beach, and almost immediately released tyre pressure as set on my tyre deflators @18psi and engaged 4WD low on my trusty '91 Nisaan Patrol wagon and locked my front wheel hubs.
To our surprise coming to a right hand bend, a huge sand dune behind big trees appeared and noticed the sand on the slope has already spilled onto the track. I stopped the car and checked it out. Hopped back into the car and started to proceed with heaps of confidence. But ... after about more than a dozen attempts to no avail, gave up but took some pictures of the dunes.
Later I saw one 4WD tray top had no problem climbing up this bend, but didn't get a chance to speak to the driver. On the way out I come across a short wheel base 4WD presumably didn't have any problem either.
Any comments as to where I've done wrong folks?