Man... I thought this to, bit it turned out I just cannot read a map to save my life. Did you grab the stake from the cave just off of the coast? If you jump straight down towards the ocean there's a little cabe on the bottom that leads straight to where it shows on the map that one looks like its "above" ground, next to the cavern leading towards the pychic gym. There's also one like right underneath the pokemon center warp space on the mountain, near where the titan was, I kept missing that one specific stake for some reason, I have no idea why, or where I was going instead, but I just kept missing it, and this was with three different maps open and a video playing...

I kid you not I spent three hours running back and forth between every location marked on the map this sub provided and I could.not.find.it.

I ended up having to go to Tony Strongstyles video on where to find the stakes before I relaized I'd been going to the wrong place all along. Maybe give him a check and see if you're making the same mistake I was making