Sulfur is not very soluble in nitrile (NBR) compounds so regular Rubber-Makers sulfur does not disperse very easily. It is generally recommended that one use a "superfine" (smaller particle size) sulfur. This is usually treated with magnesium carbonate to further assist dispersion.
For an NBR O-ring one would normally use either a low sulfur (semi-EV) cure, or a "sufurless" (EV) cure to ensure the lowest compression set characteristics.
From your description of the white coating, it sounds to me like you have sulfur blooming to the surface. The compounder could have used the wrong grade of sulfur.
If the compounder used an EV cure system, high in TMTD or other thiuram sulfur donor, and the parts are undercured, the bloom (white coating) could be unreacted accelerator.
Add to this the fact that these parts are softer than they should be confirms something is wrong in the curing system. You may be able to post cure these parts to raise the hardness, but this would not be a preferred way to go since other properties will likely be negatively affected.
Good luck!
