Problem Faced in Conzerve EM 6400 Meters

HI I am facing probelm in 11 KV HT switchboard:

I hvae one i/c feeder & 3 distribution trafo o/g feeders.

Each feeder is having conzerve meter EM 6400 meter of same accuracy class. We are facing difficulty that

KWH of i/c is not equal to (sum of kwh for distr trafo1+sum of kwh for distr trafo2+sum of kwh for distr trafo3) when plant is on full load capacity.

We are getting correct KWH values from meters as we have checked CT connection of all feeders.

Pls suggest.