Unable to sum up column in ABAP

Dear all,

i am using structure LVC_S_LAYO.

Here in my toolbar some icons like show next,summation are inactive.

Also i am using LVC_T_FCAT where i have set DO_SUM = 'X'.

When i see in debugging the value X is getting passed from my it_fieldcatalog table but still

the sum of the desired column is not displayed.what must be the problem?

Why are these icons inactive in the toolbar.?What should i do to Activate them?