My NEO Flex piston is not traveling smoothly through the brew head, or I'm feeling a lot of resistance.

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.

Note the NEO Flex's Flow Control portafilter benefits from a grind size much coarser than typical espresso brewing, as its inner Flow Control properties are already working to slow the flow of water through the brew head. 

If you are grinding too fine, this feature will be compounded too far, thus preventing water from passing through 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, similar to the reference photo below. Never apply excess force/attempt to force the lever down if too much resistance is met. A proper shot brewed on the NEO should only require about 30 lbs of force applied to the lever.