Bill brought next idea :
I don't seem to have this problem on genuine Parker ballpens, but have had it
on some Chinese copies.

The thing about these ballpens is that they survive in my trouser pockets,
whereas every other make I've tried breaks up amd usually then leaks. The
Parker design, with a strong plastic front end screwing inside the steel
part, is much stronger than the average.

So far as I am aware they are all genuine, some half plastic, some all
ss and all exhibit the same issue. The only way to get them to retract
when they jam, is to partially undo the barrel. That seems to suggest
it is the 'nib' getting jammed in the barrel.