ammonia equal zero great part of the time

i have simulated the change of NH4 with initial concentration =0.657 in each waterbody and layer. upstream head constituent concentration and downstream head constituent concentration of NH4 equal 0.657 also. sediment release rate and decay rate were set to zero. in my output files (TSR plots)the NH4 equal to zero great part of the time from the junction of two branches. it should not eauql zero in my opinion. the constituent is set to active and "CST PRIN" is set to ON for all waterbodies. the hydrodynamic is well calibrated versus observed data. i turned on the snapshot output file. the concentration are 0.0 great part of the time also. any suggestions??