Individual characters and costumes do not reward clams for unlocking them, they are part of larger collections that must have all associated items within them unlocked in order to get the reward. To check your progress on the different collections check your Facespace!
When you do complete a costume or character collection, the bonus is automatically added to your total Clam amount. Because this updates at the same time you get the congratulatory message, and because clams are not dropped on the ground, this can look like the clams were not rewarded.
When you do complete a costume or character collection, the bonus is automatically added to your total Clam amount. Because this updates at the same time you get the congratulatory message, and because clams are not dropped on the ground, this can look like the clams were not rewarded.