Poorly lupin plant

Lizzybusy

I have 5 lupin plants all growing nicely except one where the lower parts of the blooms have dropped off. They have not been eaten by bugs as the little blooms are all over the leaves. Could the plant be short of water? The leaves are not droopy so I thought not. But apart from the position in the garden - perhaps a bit less light - I do not know what has happened. I am going to cut the blooms off and wait for a second flush. Any ideas?