[edit] Never mind, found and defeated the boss.

I did Coldharbour a while ago, but there were some delves I never completed, so I returned there today to complete them, except for one.

I've re-entered Cave of Trophies delve several times, even reloaded the game several times, but Gnilokelo is never spawning, so I can't complete the delve.

There's a 2016 thread about this same problem, which said that the game restart worked, but it's not working for me.