all chicks getting tail feathers except one

Question..I know it may still be to early to tell how well (our unwell) we picked from the bin of barred rock chicks. But they're about a week old and I've noticed they've all started to get tail feathers besides one chick. This same chick also hasn't develop developed as much of its wing feathers as the others.
I'm still totally stumped on the head dot and leg wash...this chick doesn't seem much different in that aspect from some others.
Any guesses? anyone with experience with this breed notice that one gender develops tail feathers before the other or is this just a fluke?

 

chick in question is the one on the top (above picture)


chick to the right (above)

 

Do those help?

 

Thanks! Either way, I know we've got what we've got and I'm loving watching them grow.

 

Just for fun/curiosity...here are the little peepers!!

 

Aw! How old are they? I'm hoping they're pullets too