What fraction of cord blood donations get accepted?

In the United States, about 80% of cord blood donations are discarded. The primary reason is that the collection volume is too small. Public banks exist to provide transplants for patients throughout the world, and hence they only store cord blood donations that are big enough to transplant a large child or small adult. On top of that medical requirement to only accept the bigger collections, in recent years public banks have raised their storage thresholds in response to economic pressures.

Reference:
Magalon et al. 2015 Banking or Bankrupting: Strategies for Sustaining the Economic Future of Public Cord Blood Banks PLOS ONE doi:10.1371/journal.pone.0143440