Using a Soxhlet is like making tea dozens of times over and over with the same leaves using fresh solvent each time. You get everything out of the plant this way. But instead of using fresh solvent, the Soxhlet system recycles the same solvent over and over by automatically re-distilling it for each cycle. With each cycle the solvent in the boiling flask gets more and more concentrated with extractives from the plant material until eventually everything that can be extracted is extracted.