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I would give it some more time. Warm season grasses begin to grow much later in the spring as air and soil temperatures increase, so I wouldn't write off your perennial grass just yet!

For more information on pennisetum, please visit the following link:
https://www.gardeningknowhow.com/ornamental/foliage/fountain-grass/tips-for-care-of-fountain-grass.htm

I would give it some more time. Warm season grasses begin to grow much later in the spring as air and soil temperatures increase. This is especially the case after a cold, hard winter.

For more information on japanese giant silver grass, please visit the following link:
https://www.gardeningknowhow.com/ornamental/foliage/japanese-silver-grass/japanese-silver-grass-care.htm

I would do a search for ornamental grass producers and nurseries.

Planting ornamental grasses myself, I have found many varieties quite slow to start off.
Continue to water the new plants, if you do have loss of plant life, most reputable garden centers will replace a plant.
Here are a couple of good articles.

https://www.gardeningknowhow.com/special/children/storybook-gardens-for-kids.htm

https://www.gardeningknowhow.com/ornamental/foliage/ornamental-grass/sensational-borders-with-ornamental-grass.htm

I am just north of you in Southern MN and I agree, we have not been warm enough for many plants.

Is the soil well draining or are the plants staying soggy? If the ground is not draining the plants could have root rot.

Generally ornamental grasses and Pampas Grass are easy to care for, with little issues.

Give them more time. Watch carefully for signs of insects or disease as damp conditions can invite problems.

https://www.gardeningknowhow.com/ornamental/foliage/ornamental-grass/no-plumes-on-ornamental-grass.htm
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm

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.

Big bluestem grass seed can be purchased or harvested right from the seed heads. Collect seed heads when they are dry in September to October. Place the seed heads in paper bags in a warm area to dry for two to four weeks. Big bluestem grass should be planted after winter’s worst has passed so you will need to store the seed. Store it for up to seven months in a jar with a tightly sealed lid in a dark room.

For more information on big bluestem grass, please visit the following link:
https://www.gardeningknowhow.com/ornamental/foliage/big-bluestem-grass/big-bluestem-grass-information-and-tips.htm