Down to pollination, and to some extent, the variety of the tree - if its Corylus avellana, common hazel, this is particularly true. Other hazels grown particularly for their nuts, and which fruit better, are varieties of Corylus maxima, such as Kentish Cob.

I have a long Hazel hedge and in 14 years it has never had a single nut on any of it. I think that some forms are sterile, my hedge does not even produce catkins.