Neil,
I don't understand why you did not place the pivot hinge as shown below??
Lowering the pivot point would then in turn keep it on the same plane as your applied loads.
As it is now, your putting "rolling" forces on your entire load setup, which in effect, causes shear forces across your composites that are under test.
Yes, I understand that the pin can roll, but the load "center" shifts in position with higher load forces.
If your trying to accommodate thicker samples, then you could use differing thickness shims to accomplish what is needed.
Jim.