I have used parting tools on hundreds of pieces, so I know the basics about approach and widening the cut to prevent binding. I am working on a set of goblets that are 3.5" wide at the foot. The wood is very dry kiln-dried ash. Ash has sort of a rugged grain where the hardness of the light and dark rings varies quite a lot.
As I get more than about an inch into the wood, even with a double-wide cut, the leverage changes and it throws the tip downward, catching the parting blade between the piece and the tool rest. I have experimented with high, middle, and low cuts, delicate and firm grip, fast and slow cuts, and fast and slow spinning.
The tool I am using is D-Way Parting Tool (new) with a D-Way 8" handle and sharpened on a CBN wheel.
To complete the parting I started with the D-way, switched to my old wider parting tool which has been used past the hardened portion, and finished with tenon saw. Any ideas or basics I am missing.