Hi Rod and All.
[I think we met at the 3D Champs when you were admiring my other half's ash zigzag bow....]
>If one gets too loose through heavy use I sometimes line the nock with a layer of glued on tissue paper.
If my self nocks get too loose, I dip the nock end of the arrow into a pot of thin varnish, and then suspend the arrow by the pile end. This not only tightens the base of the nock slot, but forms 'lips' of varnish at the extreme open end of the arrow nock, holding the arrow on the string. I use subsequent dips like this, with plenty of drying time between, to ensure a good fit to my string. However, taken too far, this method can form a snap-like nock, which I'm not so keen on though. Experimentation rules.....