If its a mature, healthy tree and its not producing nuts, then it might need a good thrashing!
I recall an old saying that I picked up somewhere in the Cotswolds: "Women, dogs and walnut trees....the more you beats them the better they be's!"

While I don't advocate beating women, nor am I in favour of cruelty to animals, I have heard that literally whipping the trunk of walnut trees encourages cropping.
I understand that it works much like bark-ringing, to reduce the tree's vigour, and "threaten" it, so it will produce nuts. This general approach works well with obstinate apple trees like my Bramley, and should be benificial to a vigorous tree like a walnut, regardless of variety.

This is probably as good a time of year as any to grab a cat-o-nine-tails, and take out all your frustrations in a fit of arboreal flagellation.

regards,
Marty, Co. Cork.