Question

Why might a photon arriving at a photodiode not give rise to photocurrent?

Answer 1

for a given metal there is cut-off wavelength λ, such that a beam of light of wavelength larger than  

cut-off wavelength λ doesn't cause the electron to be ejected from the metal however strong be the source

2nd explanation  

when a photon hits the electron it transfers its whole energy to the electron if this energy is less than the work function of the metal it will be not ejected so no photocurrent  

hope this helps if not let me know in a comment