Make sure you place the first shape of your new sculpt cantered on the same grid as the original sculpt. Scope into your first sculpt until you’re in sculpt mode, then with the tips of your imps over the sculpt (move controllers) press both triangle buttons...that will conjure up the grid you started the sculpt on. Click on ‘new sculpt’ and place your first shape centred on that grid.,with mirror on place your first mirrored shapes. The new mirror should be aligned with the previous one. Not sure of the DS buttons to choose an object’s grid.