The detonators will not explode when certain enemies are hit with continuous damage. The ones that I have seen to have this issue were minotaurs, knights, and wizards all of which the detonators won't explode due to continuous damage but they will explode once killed. For those enemies I tested it and it applies to any enemy tier (minor, major, ultra). I also believe this applies to Taken versions of said enemies. On the other hand, Vandals and Captains get another void detonator placed on them after one already goes off. I don't know if it was intended to reapply the detonator on the same enemy if they survived the explosion. There may be more enemies affected by these bugs and hopefully they can be fixed to add consistency to the Code of the Commander.
Yeah definitely noticing this! Running it with Doom Fang Pauldrons to get the most out of the melee and the sentinel shield (which also creates detonators) but I have noticed them not detonating on these enemies too, interestingly they are all powerful hive enemies. Maybe this is by design? Would be good if it wasn't the case though!