I would suspect that some of the clumps are getting less phosphorous than the others. Plants need phosphorous to create their flowers and seeds well (which the plumes are). Try giving the non-pluming plant some bone meal.
If this is an established planting, you may also have some mature clumps while others are still too young to bloom. Like animals, plants need to reach maturity before they can reproduce, so a young plant may not be able to create plumes like older plants can.