When doing quests for new abilities, you may sometimes notice that the enemy you need to kill sometimes won't count for the quest's quota; this may be due to two things:

1. In the same Act and Area, but in different rooms. The powers of Collaboration and Spirit-Link will give you a rough idea how far you need to be for experience to count for "everyone in the fight". Generally speaking, characters in the same room will share the experience of kills.

2. Lag while online. Sometimes it occurs (especially for bosses). The best way is to get in a few licks (damage the boss) and make sure you're part of the fight before the boss gets cacked.