It's a lot easier with a mouse... level 5 is pretty hard!! Were you using a mouse? Level 6 gives you the ability to get cells anywhere along the vein, which obviously helps a lot. Also: to move cells faster, turn off all the receptors besides the move: with only move receptors on its easier to move cells.
We are re-building the game, so we will be making the levels ... well, not easier, but the progression will be smoother!