The lack of charge to GCMS is quite possible, and worth thinking about, but there is plenty of other evidence about the oxidative nature of Mars soils that suggests you wouldn't see much organic material in a shallow sample.

As for the clumping, it isn't electrostatic effects per se. but Van-der-Waals forces within very finely ground particle aggregates. On Earth, wheat flour is a great example. It forms clods and clots even when it's ultra-dry and even when you take care to avoid electrostatic effects. The problem is that very fine powders are ground so fine that the individual mineral grains and molecules are torn apart and unsatisfied charges are "hanging" on those surfaces.

Water acts to kill all these charges (with a complication of flocculation of fine powders, and detergent/olefin monolayers), but in general the activity of liquid water over time will "pacify" a clumping powder (often by cementing it into a permanent solid, but at least you won't be mislead)