Problem on MPI: About Non-Blocking Collective operations

The structure of my code is,

//part1
if(i>1){
          Compute1;
        }
//part2
if(i<m)
      {
           Compute2;
         MPI_Allgatherv();  //Replaced by MPI_Iallgatherv();
       }
//part3
if(i>0)
     {
         Compute3;
         MPI_Allreduce();
     }
part4
if(i<m){
         Compute4;
         }

Collective operations in part 2 is the bottleneck of this program.

I replaced "MPI_Allgatherv()" by the NBC "MPI_Iallgatherv()" in order to hide the collective communication by part3 and part4. But part3 and part4 take much longer than before. What do you think is the cause of this problem?

Thanks!