Editable subgrid funcionality

I have the account entity and two custom entities, one of them called Position Order and the other one called Position Order Item. The entity Position Order is a container for the Position Order Items.

There is a sub grid on the Account form for Position Orders and on the Position Order form there is a sub grid for Position Order Items. I can edit the grids when I expand them using that button on the sub grid, but the sub grid itself does not let me edit from the parent form.

On the form, I have already enabled the controls for the editable subgrid. Is there something I am not doing?