The knob on the top of the shock is for compression adjustment... It doesn't change anything other than the speed at which the shock will travel. You also have an additional adjuster on the bottom of the shock, that threads in and out to control rebound. The rebound adjuster, by design, will also create added compression resistance. Essentially it is a "bypass" controlled by a needle valve that flows both directions: Compression, and rebound.
Don't add anymore nitrogen to the shock, as it won't do anything. Cranking down the preload will add some, but like Possum said, you should replace the spring, if you want to do it right.
There isn't anything at all you can do with the front shocks, except adjust the preload.