"newshound" wrote:

Are you sure this is not caused by a buildup of dried grease and/or
cementatious matter from drilling? May be worth giving the chuck a good
rinse in solvent, followed by your preferred oil spray. May also need to
poke in the hole with a brush and/or jiggle any SDS bit around in the
chuck to help clear it.

I did try that in a half-hearted sort of way; I'll have another go. But the
fact that the non-locking happened suddenly and that up to then the
twist-and-lock had felt extremely smooth and positive does make me think
that something's broken or possibly misplaced. I believe that one or more
spring-loaded ball bearings are involved, is that right?