I tied it to the slate with monofilament. Used a couple of extra rocks on top of the slate for extra weight. It’s staying down now.Large pieces of wood, especially those that are dry all the way though can take upwards of 6months to a year to fully stay down on their own...
I’ve had pieces that took over 8months to fully submerge in the past...
At least I can scape around it now. Thanks guys!