You can try switching to a higher protein feed, such as flock raiser, as 16% is the minimum protein needed. and provide shells. Not every chicken across a breed lays what 'they say they lay.
Grit is granite pieces for digestion, oyster shell is for calcium. It's not the same thing, if your chickens run around outside, they usually don't need grit. Sussex tend to lay medium to large eggs. What do you feed them?