Prior to using new nappies for the first time, wash once with detergent in water of any temperature. They can be added to other laundry, ideally light items to prevent picking up colour run.
Bamboo and hemp will build absorbency over time change more frequently to prevent leaks from occurring.
See the Why Am I Getting Leaks? for methods on how to prevent them.
Why does prepping new nappies make such a difference to user success?
The surface area of the fibers increases over time with washing (ie friction), the fibers become looser and micro piling occurs. The greater the surface area, the more absorbency potential  .
See the Adding Absorbency page for absorbency measurements of various flats, prefolds and inserts.
 L. Quaynor, M.Takahashi M. Nakajima 2014. Effects of Laundering on the Surface Properties and Dimensional Stability of Plain Knitted Fabrics
 C. Ganser, P. Kreiml, R. Morak, C. Teichert 2015. The effects of water uptake on mechanical properties of viscose fibers