If the OnTriggerEnter2D method is on your explosion sprite, it could be triggering for the first entity it kills but is not leaving that entity before hitting a new one (So it doesn't fire a new OnTriggerEnter2D).
You could move the trigger event onto the entities that are being hit by the explosion - that way they are all monitoring their own OnTriggerEnter2D event.
If you want to leave your trigger on the explosion, you might be able to use OnTriggerStay2D - this will fire every frame that an object is detected within the explosion collider. However, this might still just keep returning the first entity that triggered the OnTriggerEnter2D, so not sure if that would work.