According to http://fallout.wikia.com/wiki/Oh_my_Papa notes "There is a bug in this quest with regard to Melissa's support making it impossible to complete the "Oh my papa" quest if you speak to Melissa first and do her "Don't Make a Beggar of Me" quest. It does NOT matter if you turn her in, or if you give her the chems, she will not be at the Great Khan Encampment for you to talk to. " . How do I correct this problem in the PC?

I ran into this as well. I found Melissa and got her chems, then she went missing during the Papa quest. The quest marker points to the spot she normally shows up near the quarry. This does break the Papa quest. But I found I could continue helping the NCR by doing the Major's quests at Hoover dam by...