In my map, when the hero dies it gets respawned. There's a location called MediumFollow which constantly is centred on the hero unit. Units for Player 3 who are inside MediumFollow are ordered to patrol to MediumFollow once ever second. Before the first death, the units for Player 3 attack correctly. After the respawn, they never attack the hero unit. The hero unit is not invincible, nor is it invisible. Player 3 is set to the Enemy of Player 2 (the hero unit player). Why are the Player 3 units no longer attacking the hero?

By the units are never attacking do you mean they are no longer patrolling towards the enemy, or they are longer attacking even when close to the hero unit? If they are no longer patrolling to your hero per your triggers, it could be that the location is no longer being centered on the new hero unit for some reason. If you were to upload your map it would be a lot easier to figure out because that shouldn't be happening under normal conditions.

One of these options:
1). Something (accidentally or otherwise) changed the alliance of P3 towards to hero.
2). Some other order is interfering with the units attacking the hero.
3). No order is being given to attack the hero.
4). The correct order is being spammed for some reason instead of once per second.
5). The order-location is not in the correct place on the map.

Obviously without the map we can't tell you explicitly what trigger is causing this, but I still find it helpful to consider the possibilities as above when debugging triggers, maybe you will too.

Huh. The unit is being created invincible...I already checked for that but it's come back again...I'll have to fix up the properties for that. Sorry for taking everyone's time ;o this can be closed.