I think we are going to need more facts before being able to help you, OP. There are many reasons why your birds could be dying but we need some particulars:
Weather~ Climate where you live?
Housing~heated, unheated, ventilated, unventilated
Feed~how much? how often? what kind?
Water~continuous? Are you keeping it thawed and fresh?
Bedding~none? deep litter? If so, what type? Are you turning it and keeping it fresh?
General appearance of your birds before death?
Appearance of your remaining birds?
Feed consumption?
Activity levels?
There are many who order meat chicks and they don't die, raise meat birds and they don't die, successfully process healthy meat birds and not notice any health issues at all.
Help us to help you by answering these questions?