I’m very confused by it as well.
Cora had a “spark” of magic that came from emotions which is why once she tapped into them, she was able to spin quickly.
Regina had a “spark” but was held back for a long time by her love of Daniel and wanting him back. She had to be broken first.
Zelena has more than a spark. She has a big ol’ wam bam punch. Baby Regina wasn’t doing any such thing, so far as we know. There is something MORE powerful about Zelena and I think the answer rests with her father.
So genetically:
magical father + magical mother = super magical baby
magical mother + non-magical father (or vice versa) = magical baby but delayed.
Cora’s mother or father must have had magic in them as well, which is why it passed down. I’d say mother because so far it’s tradition that the women have it.