Impatiens are susceptible to viral, bacterial, and fungal leaf spots, or they could be necrotic spots from fertiliser burn or severe overwatering.

If the older leaves go yellow and drop then I would suspect overwatering. If the newer leaves go yellow then I would think you need more fertiliser. Buds will drop because of overwatering.

Are these indoors or outdoors? Indoors Impatiens will get spider mites, which initially cause mottling of the new leaves, followed by wilting and leaf drop. You would be able to see fine webbing between the leaves at that stage.

Thanks for your response. They are outdoor impatiens. I think it's mainly the older leaves that are dropping. I'm trying to let them dry out some and will see what happens.