When chicks are vaccinated for coccidiosis you should not feed medicated starter. Feeding coccidiostat to vaccinated chicks counteracts the vaccination.
Other then that, since you've had them such a short time, it may be that these have just been some weak chicks that were too stressed or got chilled in shipping. It is common to loose chicks during or right after being shipped. Especially chicks shipped during cold weather. Getting chilled kills chicks fast and even though most hatcheries put a heat pack in the box sometimes it's just not enough in very cold weather. Weak or stressed chicks often just are not strong enough to bounce back.
Edited to add: If you haven't done so already you might add some electrolytes such as Save-A-Chick to their water, or some Nutridrench, to give them a boost.