How old were these guys? They didn't have the right size crests, so if you had shown them, they would not have done well. Here is a picture off of the ASBC site of a Buff. Take note of the crest and tail:
This one (below) has an even bigger crest... but she is lacking cushion and fluff in her tail.. Her wings are also loose and hanging low.

But her crest is beautiful!
Your birds were either too young and hadn't developed their crests yet, which is possible if they were just 2-3 months old... or they just weren't going to meet that potential. Anyway, your goal is to have the look of the first Buff-- that nice chest that bumps out, and nice cushion and tail that stick up nice and soft (you don't want a tail that slopes down), with a nice round crest. You want everything to be round.
ETA (edited to add) I am very sorry to hear that your little ones died. Very sad indeed.