In reply to petegunn:
Thanks for all the advice, everyone. To clarify:
I normally hang with shoulders engaged, elbows slightly bent. (I don't tend to hang in different position, though - maybe I should?). I don't hang off my skeleton because I had the impression this was bad form.
If I do repeaters, it's my forearms that burn and give in first. Sometimes, with a half-crimp, it's my index finger that uncurls first. But, if I'm hanging a smaller hold or with weight, usually it's my left shoulder that gives in first - and it feels like it's somewhere on the shoulder blade. I struggle to keep the shoulder engaged and it wants to head toward hanging off my skeleton, so then I stop. For example, if I do 4 reps, perhaps on the 4th one my shoulder starts to unclench before the set time.
More generally, repeaters leave my forearms and fingers feeling a lot more tired than max hangs, which I usually feel in the shoulders - e.g. the next day I might struggle to do a pull-up and decide it's better to rest.
Thanks again