Why are their tests normal?

Because no blood test or scan shows fibromyalgia. Tests are done to rule out other conditions, so normal results are expected, and they don't mean nothing is wrong.

The diagnosis is clinical: a doctor listens, examines, and arranges a few basic blood tests to check for other causes. Sometimes X-rays or other scans are done too. Finding another condition doesn't rule out fibromyalgia, and fibromyalgia doesn't rule out other conditions.