I am suffering the exact same issue as you did, but I found this problem was already exist in 2016, and they haven't solved it right now!
I did send a referral link to my mom's email address, and she did sign up using the link, however the travel credits in her account is 0!!!! But in my referral list, it did change my mom's account from"hasn't signed up yet" to "hasn't started up a trip". And then I thought is was likely the travel credits will show up when I submit the reservation and pay for it. So we booked a room in bangkok, but after we paid for the room, nothing about travel credits come up. It is inapproprate to deceive users like that.
I called customer service and one staff told me she has delivered my issue to the upper staffs in Airbnb, and they will respond asap. However, I haven't received any response from them!!!! Will update here if these is any progress!