When the president relapsed with cancer again, why didn't they use Hera's blood again? Seems like it worked once, I would think it should work again.
The issue was that in the original version of the episode, Roslin was cured by using fetal stem-cells from Hera. When the cancer re-emerges, those stem-cells are no longer available.
The mention of stem-cells were cut from the final version of the episodes, but Ron Moore explained it in the commentary track.
From Battlestar Wiki:
In the podcast, Ron Moore stated that a longer, more detailed explanation of the curing of Roslin's cancer using fetal hybrid blood was filmed - explaining that it was the fetal blood stem cells from the baby that were the actual cure - but Moore was afraid that this would be seen as technobabble and criticized by fans, so it was edited down to a truncated version. Ironically, this resulted in widespread complaint in critic reviews and fan message boards that there was too little explanation in the final version, and uses made up science in favor of real one.
While omitted from the actual show, syfy.com's profile for Laura Roslin also mentions the stem-cell treatments.
Though she has risen to the challenge of leading the fleet with class, confidence, intelligence and stoicism, she also battled breast cancer before it was sent into remission by a stem-cell treatment from Hera, the human-Cylon infant.