There is a slight difference. 来る is an action verb. 来てる is a stative verb. If you are waiting for someone at a certain place for a long time, you can say まだ来ない？ or colloquially, まだ来ないの？ If you get to a certain place and find out that someone hasn't arrived yet, you can say まだ来てない? or colloquially, まだ来てないの？ You can also say 来ていない instead of 来てない. But I think 来てない is more colloquial. But, ている ( or てる) has many functions. For example, "I'm coming now" or "I'm on my way." → 今向かっています。 ている is used for like present continuous. I'm not a professional teacher. Please read the other answers if you like.