For review, spin recovery will become more difficult/impossible if the rotation cannot be stopped, and the nose not substantially lowered as recovery is initiated.
Rotation stopping may be degraded because the rudder is ineffective (perhaps blanked), or inertial forces of rotation too great for the rudder to overcome (engines, fuel in tip tanks).
Lowering the nose will be affected by elevator effectiveness (blanked or disturbed air off wing, fuselage or fin/rudder), or aft C of G elevator cannot overcome.
During spin testing I flew in a modified Cessna Grand Caravan, the aft limit configuration showed and aircraft which was not eager to recover. It did, but it made you sweat, as the nose could not be forced much below the horizon. Watching the video of the doomed 421 reminded me of having difficulty getting the nose down to initiate recovery. When I was doing it, I had many more thousand feet under me....