The stick makes sense because it's caused by the agitation. I've always used Paterson tanks with inversion processing of Colour and B&W. If you stir water you get a vortex, the liquid level dips in the center and rises at the edges, this can happen when you use the stick method of agitation.

The reason I said insufficient developer was because there's clearly under development around the sprocket holes, but Herzeleid has almost certainly identified your problem. With inversion agitation a lot of the Paterson tanks use a collar to stop the reel shifting up the column, if you haven't got one choose a spiral that's reasonable tight fit on the column, they do vary a bit.

Ian