The worldboss Zalkenrys (107213) does not respawn after killing it once after servers are started.
This is because Zalkenrys (107493) has a respawn of 24 hours and all other Zalkenrys IDs (107213, 107417, 107481) have no respawn. The problem is that the NPC which needs to respawn is not 107493 since Zalkenrys' respawn needs to be triggered by the flying NPC object.
Please change the respawn of the correct ID to 24 hours so that Zalkenrys will respawn.