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Use a fertilizer with micro-nutrients. This article will help:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/caring-for-and-landscaping-with-yuccas-outdoors.htm

It may be aphids or scale. I would treat the plant with neem oil. It is effective against most damaging pests and is safe for people and animals. Here are some articles that will help you:
https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm
https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Fertilizing might help, but typically yuccas don't need a lot of fertilizer. It may be pest damage or over watering. I would treat the plants with neem oil to get rid of the pests and check the soil around them to make sure that it is not too soggy. If you have gotten a lot of rain lately, this could also cause this.

If they are indoors, it is most likely over watering. If they are outdoors, it is most likely a lack of nutrients.

Though they are known to skip years now and then, adding phosphorus, like bone meal, to the soil will help encourage blooming. This article will help with propagation: https://www.gardeningknowhow.com/ornamental/foliage/yucca/propagation-of-yucca-plant.htm

Yuccas bloom inconsistently. It is hard to say how often it will bloom because of this.