[Jac Mac]

Make sure the shifter assy has dry lube or similar between the individual shifter plates, we have found similar issues in Jerico 4 speeds where the plates drag against one another enough to prevent the detent between the 1/2 & 3/4 shifter rods ( internal in the box ) to line up exactly during the 3/2 or 2/3 shifts and therefore the shifter feels like its hitting a brick wall, yet with the engine off it shifts fine.
Also as Chas mentions a couple of posts above the stop nuts on a Hurst must be adjusted correctly as well- flexing shift rods can also cause issues.