There are some people who believe that adding more pressure will improve the felting. This is a misunderstanding probably circulated by manufacturers of old style felt rolling machines or a throw back to the old steam pressure for making felted hats - although the process was a little more complex than just adding steam and pressure.
Pressure does not create felt. There is a video below explaining why this statement is true but imagine for a moment you put some wool in a vice, or under a huge weight – leave it there as long as you want, the pressure will not turn your wool into felt.
Agitation is what causes felting to occur – the rubbing and meshing of the fibres with each other.
So the consideration of how to make felt comes down to a matter of how to agitate your fibres and layout.
Squashing your wool with ever more pressure is not the solution. The solution is in making the fibres move relative to each other in the felting process.
Bubble wrap is a great and cheap solution. When you felt using bubble wrap the bubbles move back and forth and deform in shape as you roll on it (or in it). If you are using mechanical means to roll, such as The Gentle Roller, then you need to replicate this slight movement of the underlying material in order to agitate your fibres in a controlled fashion (you don’t want them over agitated to the point where they cannot adhere to each other).
The super-soft idle roller set is the ideal solution for fine nuno felted items. The squishiness of the roller compound is just sufficient that in the rolling process the fibres are agitated together as the rollers work back and forth under the idle rollers thus creating agitation.