  Possible reasons for the non-detection of absorption in the metastable
HeI(2^3S) line at transit observations of warm Neptune GJ436b, in spite of the
well pronounced strong absorption features measured earlier in Ly{\alpha} for
this planet, are investigated. We perform numeric simulations of the escaping
upper atmosphere of this planet and its HeI(2^3S) triplet absorption with a
global 3D multi-fluid self-consistent hydrodynamic model. By fitting the model
parameters to the lowest detection level of absorption measurements, we
constrain an upper limit the He/H abundance three times smaller than the solar
value. We demonstrate that neither the significant changes of the stellar wind
related with possible stellar coronal mass ejections (CMEs), or possible
variations in the stellar ionization radiation, nor the presence of heavy trace
elements have crucial effect on the absorption at the 10830{\AA} line of
HeI(2^3S) triplet. The main reason of weak signature is that the region
populated by the absorbing metastable helium is rather small (<3R_p), as well
as the small size of the planet itself, in comparison to the host star. We show
that the radiation pressure force acting on the HeI(2^3S) atoms spreads them
along the line of sight and around the planet, thus further reducing peak
absorption.
