Trouble with Plane

Trouble with Plane

I am trying to create a 3D plane on the pictured ledge. I was completely successful creating one on the opposite side, they are exactly the same size. Probe has plenty of clearance, I am not running into the table. I even tried adjusting the points right to the middle of the ledge and it simply will not allow me to create? Like I said, the opposite side of the ledge created a plane perfectly with no issue?