Geezer here: Well durn, I was in the middle of writing a reply to the notch-lock problem, when the computer ate my letter. I'll try again.
Clinker had problems with his notch and push-pin lock.
Push-pin locks are pretty primitive and they don't work all that well, particularly with stronger prods. That's why all crossbows don't use simple notch and pin locks. They tend to have creepy releases, often misfire, beat up the butts of your bolts, and devour string serving. So what can you do short of going to a more sophisticated lock?
1. Replace the release lever with a stiffer one. That will take the spring out of the release. A longer lever or changed fulcrum-point will make the trigger pull easier, but may make it even longer.
2. Smooth and re-face the notch, to make the string slide easier. (a bit of ivory or Delrin on the face of the notch will reduce friction.
3. A shallower notch will reduce the distance that the string has to slide may improve matters, so long as you still have enough depth for a secure lock.
4. A low-friction serving for the string... like monofilament, may reduce the drag on your string.
5. A thinner bowstring, carried higher in the notch might improve performance over all. Ideally, you want the string to push the bolt at its middle. So if your bolt is riding in a groove, you could probably get away with reducing the diameter of the string a bit and still get a centered push.
6. If you don't have a 'string snubber' installed to keep your bowstring from jumping over the butt of the bolt and misfiring, you should consider fitting one. They're simple to make and effective. All you need is some sort of plate that fits over the notch. There should be enough space between the plate and the bolt-groove to pass the string, and if the snubber is just a bit springy so it will serve as a bolt-clamp, so much the better. With the snubber in place, the string should not be able to leap OVER the bolt and cause a misfire.
7. I doubt changing the diameter of the push-pin would improve performance.
Does anybody else have anything to add? You have the floor. Geezer