Is the brawl over yet? If not, then please wait until the brawl cycle is over.
Is your brawl stuck? If so, then please refer to: Our brawl is stuck! What do we do?
Did you submit all your brawls or flee from all your brawls? In order to get full merits, you need to submit and not flee from at least 75% of your brawls (It is okay if you lose the battles). Otherwise, the amount of merits you receive is proportional to the amount of brawl battles where you submit and do not flee. If you do not submit any brawl battles or if you flee from all your brawl battles then you will not receive any merits. (But if you submit all your brawl battles and lose all your battles but not through fleeing, that is okay and you will receive full merits).