Why am I missing V-Bucks from the Fortnite Battle Pass?

Free and Premium Battle Pass share the same progression. If you earned V-Bucks from the Free Pass before purchasing the Premium Battle Pass, those V-Bucks already count toward the season's 800-V-Bucks total.

If you earn 200 V-Bucks from the free pass and then purchase the premium pass, you will be able to unlock an additional 600 V-Bucks, bringing the total to 800 V-Bucks earned.

To learn more about the Battle Pass please refer to: What is the Battle Pass? Where can I learn more?