 Solution

One possible cause is a different indium gallium nitride thickness or indium content that shifts this wire's emission away from 420 nm. The detected intensity at one wavelength then falls even if its integrated radiative efficiency is unchanged. Since $hc/(420\mathrm{nm})\simeq 2.95\mathrm{eV}$, composition, strain, confinement and internal fields can all move the transition relative to that detection energy.

A second physically different possibility is poor active-layer quality: roughness, composition segregation or strain relaxation can introduce defects and nonradiative recombination. Locally thinner or discontinuous shell regions may also permit carrier leakage.

Measure the full spectrum and integrated emission, rather than one spectral channel. Time-resolved luminescence, excitation-dependent measurements and local structural/compositional characterization can distinguish a spectral shift, changed carrier injection and an increased nonradiative rate. The single-wavelength observation alone does not uniquely identify lower internal quantum efficiency or one growth defect.