What is 300 Form?
I don't know what they sold you, but it doesn't seem like they're even the White Rocks, let alone Cornish X. The description on their website is confusing:
"Fast Growth – White Rock Cornish x
"These extra heavy meat-type birds are bred for meat only. Their strength is in their...
Adding to my comment above: I don't know about other hatcheries, but I always order my Jumbo Cornish Cross chicks from Cackle Hatchery. Always healthy, robust chicks and I've never had a single death from whatever issues this breed commonly suffers from. If I remember right, average weight after...
Can you post any photos of them? Sounds like they weren't Cornish Cross, which is likely what you've been raising before, by your description of them (lying around, eating lots, no flying, butcher at 8 weeks).
You ordered from a different company this time. Did you contact them? I wonder if...