about MSD calculations

Dear all lammps user’s
i have tried to calculate MSD for one H atom diffusion in my 433 SC lattice
i have used the next command:

compute msd3 H msd
fix msd H ave/time 1 1 10 c_msd3[1] c_msd3[2] c_msd3[3] c_msd3[4] file msdH.out

but when i get the graph of MSD it appear don’t the same as in ordinary case and MSD value increase in first time than return to 0 value than increase and saturated
the run was make for 100000 iteration with 0.5 fs time step
so can any one comment this
best regards

If the MSD is saturating, then the atom
isn't diffusing. Could be lots of reasons for that.