Time and the speech coming in. I think its directly related to the receptive issues (or at least for us). When my chid got slightly more responsive we kept reminding him to look at us... Eventually it got more natural. When he was talking more, he turned it around on us and told us to look at him when we were not.

At 20 months, depending on the severity of the delay, he may catch up in a year, or several years. My child did not really start till 4 and it took a few more years after that. If your child is a later talker, know they can still start talking even at 4-5.