There are so many factors that can affect oxytocin production, physiological as well as psychological and emotional. Not everyone experiences a post-birth "high." Sometimes it takes more than oxytocin for a mother to feel positively about her birth and she might have to work harder to bond with her newborn.
There may be an underlying issue she's not aware of. Did this mother have any sort of fright or trauma during the birth? Was it a long or exhausting birth? Does she have any negative feelings about the experience (it might be uncomfortable to share if everyone else in the room experienced a birth high!) Was anyone in the room afraid or discomforting to her while she was laboring? What were her feelings about the pregnancy and birth to begin with? Did she have any risk factors beforehand that might affect her hormone balance (history of depression, unresolved loss, sexual abuse history, relationship strain, stress, etc.)? Does she have the kind of support in place now to help her feel like she's doing a great job at being a mother (even when no one else is around)?
If this was someone I knew and they were concerned about it, I'd encourage them to visit a postpartum support group and/or talk to their care provider about what they're feeling.