It's really hard to guess why she'll eat a later meal but not eat
the first meal, but if she's healthy & gains weight and happy, I
wouldn't worry about it. I adopted a 2nd dog (Maggie) at 4mos
old and she prefers to eat only one meal a day, in the evenings.
My other dog, Smokey, wants to eat once in the morning & then
again at night. They both weight about the same now, so they
must be eating enough. When Maggie was younger she did eat
two meals, but stopped doing that months ago. She's happy and
healthy & so is Smokey. Go figure.