Hey, I had this problem as well. I had three active exotic bounties in my inventory. Then I got a fourth one and it would not let me hand it into Xander. I believe this is your problem. I completed one of my other exotic bounties and then I was able to turn it in and get another exotic bounty. The bounty I finished was for invective. Then the new bounty was also for invective. It did not give me any options, just the one came up. I hope this wasn't too confusing and helped you a bit.