Thanks for the feedback! It was on a weekend (I booked on a Sunday), so perhaps that was the issue. And I was checking the Delta seating chart myself (no "6 seats left" notification). Although, I had actually ended up calling customer service to book my flights, since I payed for 2 seats with my miles and 2 seats outright. I had mentioned to the SkyMiles customer service agent that I noticed there were limited seats left, and he didn't correct me. Maybe just an oversight? Again, thanks for the responses. Not much I can do about it now, so live and learn, right?