The effectiveness of the Claw.
It seems to work on and off. He mentions some theory about how maybe it works when he was the one who caused the injury (like the man-ape) but I don't think we ever got a full explanation.
I think he tried it on Jolenta to no avail.
The two times I remember Severian attempting to heal and it not working were Jonas and Jolenta, both of whom were in some way "artificial" or "artificially enhanced" people. He's generally able to heal "normal humans," albeit with some limitations: he can't resurrect a whole battlefield, for example.
This is indeed an example of unreliable narration, in the sense that Severian at this point doesn't really understand what the Claw is and why it "works" for him. The answers are made clearer in Urth of the New Sun.