The plunger in the French Press doesn't go all the way down by design. This is to ensure that when you plunge, the coffee gets filtered but not squished; else, there'd be no space for the grounds to settle after plunging. If pressed to the bottom, it would force the fines to seep through the mesh and result in a muddy cup.
However, this gap does not affect the extraction process; all flavors and coffee oils are passed into the brewed coffee.