my wellsummers keep dropping dead?

[obxbum]

so i started with 9 wellsummers, now i'm down to 5. i had one die at 3 weeks. then two at 4. then one now at 8 weeks and one looks like he's not far from it. no signs of any problems that i can see. no feathers plucked out, no runny noses. dead bodies in perfect shape. i have three other breeds in with them. they are all fine so far. any advice?