Black bamboo

My black bamboo while growing up to 8 foot high now, seems to lack leaves. This seems especially noticeable at its top. Does anyone know if it's not meant to be as dense? It's growing in a part of garden with sun till midday then mainly shade.
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The leaves look so much healthier which makes mine look a little sickly.
I'm thinking it may be not enjoying the opposite. My soil is clay based and not very draining. I'm wondering if that might be why. I shall check with my local nursery tomorrow.
Thank you
I have a black bamboo clump now between 15 and 18 feet high and I can see it through my bedroom window on the first floor. My soil is clayey and moist and the clump of bamboo is on the north side of my house so it doesn't get too much sun. It loses its leaves regularly without looking bald and I assume this is natural. I cut the black canes and use them as plant supports in the garden. I have the golden version in the back garden but it is only 10 feet high as its younger than my black one.