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Be cautious about planting near black walnut. They emit a chemical from their roots called juglone. This chemical is toxic to most plants. Here is an article that will help:https://www.gardeningknowhow.com/edible/nut-trees/black-walnut/black-walnut-compatible-plants.htm

Technically they can cross. Label branches that you purposely cross pollinate, and plant seeds outdoors in pots. Label everything. Some of them can end up being a cross between the two.

These articles will refresh you on the care requirements.
Check soil moisture levels.
A soil test can help you determine if soil amendments are needed.

https://www.gardeningknowhow.com/edible/nut-trees/black-walnut/planting-black-walnut-trees.htm

https://www.gardeningknowhow.com/edible/nut-trees/walnut/planting-walnut-trees.htm

Several cultural factors can influence flower production, fruit set, and fruit maturation. Because female flowers are formed within the dormant buds when the current year's nuts are rapidly enlarging and filling, the immature flowers must compete with the nuts for carbohydrates produced by the leaves. Cultural practices such as fertilization increase the number of new flowers formed in the buds.

Water and fertilizer is about all you can do. Wait and see.

Walnuts can be quite aggressive in many climates. They will grow on their own but they will become quite large quite fast.

The toxicity applies to the soil, mostly. Although the nut's flesh can be mildly irritating, it won't cause any harm to animals or humans. This toxin affects other plants in the area. It will have a large impact on tomatoes, potatoes, and anything else solanacious, as well as many other plant species.

This article will help: https://www.gardeningknowhow.com/edible/nut-trees/walnut/growing-butternut-trees.htm