With use, the piston's o-rings will naturally lubricate thanks to the exposure to coffee oils. However, you can expedite this process by applying a light application of food-safe oil to the plunger o-ring and wiping away the excess.
However, if the piston is not moving down through the brew head without the application of excessive force, and it feels more like a blockage than a lubrication concern, it is more likely an issue with your grind size than the machine/its components. If you are grinding too fine, water will be unable to pass through the coffee bed under normal force, thus building up this pressure/resistance in the brew head and preventing the piston from lowering.
Carefully terminate your attempt by slowly raising the lever (do not abruptly let go: the pressure built-up will result in a rebound effect, causing the portafilter and cylinder to separate) and reattempt with a coarser grind size. Never apply excess force/attempt to force the lever down if too much resistance is met. A proper shot brewed at 6-8 BAR translates to about 40-50 lbs of force on the lever.
Consider upgrading your Classic with the Pressure Gauge Kit to monitor your force application and see, in real-time, what BAR you are brewing at.