When I was being tested, they initially thought that I was fit for donation, but not a match for my husband because he has type O blood and I have type A. They eventually figured out that I was type A2, which can donate to O...but in the meantime, we figured we'd have to do paired donation. In our case this would have been difficult, since he needed an O kidney, which in most donors, could just be given straight to their own recipient. Maybe we would have gotten lucky with a chain started by an altruistic O donor, but who knows? It could have taken quite some time.
Maybe that's part of the issue at times?