Very puzzling. Have you tried contacting Burpee? Since it's their seed, they may have some insight. You plants look perfectly healthy, except (maybe) that the foliage is much lighter. That could be normal. But it could be some kind of deficiency.

Well Donna, I guess they just aren't the heat lovers that the other marigolds are. After a couple of weeks of sub-100 temperatures, they are finally blooming. Maybe it is something to do with the type of hybrid they are. I'm curious to see if they set seed. If they do, I wonder what type of flowers will develop. Maybe some selfed or crossed with some of my other marigolds.
BTG

I planted several beds of marigolds and some of them are
now blooming, much later than some. I have some large yellows, blooming and a pic of some late ones . I don't know
the name but know they are late bloomers. like these.
Happy gardening, caroline
{{gwi:22692}}From Marigolds

Hi Caroline,
Your picture looks like Scarlet Starlet. The ones I planted bloomed early with my others. The white ones seemed to be the only ones that would make a bud and stop growing. I'm glad they finally did. Here's a pic of one.
BTG
{{gwi:22693}}From Flowers

BTG, your white one is beautiful, I never did have
white ones. Well, maybe I planted mine later than usual.
I was planting marigolds and zinnias at all different
times thru the summer, I still have some just now
budding in a huge pot. I never have kept the names of
these. Thanks for looking. Happy gardening, caroline .