If the cells are not infected extensively at step 2, either add additional input cells to the flasks or wait longer for the infection to spread. Alternatively, if the infection is not homogeneous and there are uninfected regions, the cells in the flask can be trypsinized, and transferred to a new flask, without the addition of uninfected cells. Make sure that the cells are healthy enough to survive the transfer; otherwise this will result in a massive death of cells which will not generate cell-free virus.