Clark needs another compatible variety nearby to set fruit. Hazelnuts are self-incompatible and wind-pollinated. Two S-allele-compatible varieties with overlapping catkin bloom are required for any crop - matching bloom timing alone is not enough if the S-alleles clash. It flowers in early bloom, so a partner has to be flowering at the same time to be any use. Usually grown in USDA zones 5-8.
No single variety stands out as the obvious partner, but that is not a problem here: Barcelona, Butler, Casina, The Beast, and Daviana all bloom close enough to work. There are 5 more in the list below. Pick whichever your nursery actually carries. Plant hazelnut companions within 50 to 100 feet. Wind carries pollen beyond that range in open sites, but closer is more reliable.
One thing to keep in mind. Hazelnut self-incompatibility is governed by S-alleles. Two varieties that share dominant S-alleles cannot cross-pollinate each other even when bloom timing overlaps. Known incompatible pairs are flagged in this tool - verify S-allele compatibility for any unlisted pairing.
To put that into practice: at a minimum, two hazelnut varieties confirmed to be S-allele compatible with overlapping catkin bloom. Better still, three varieties covering early, mid, and late bloom so pollen is available throughout the full female-flower receptive window. If an existing hazelnut produces no nuts despite flowering, the most common cause is an S-allele-mismatched companion. Replace or add a confirmed compatible variety.