Diagnosing sacroiliac joint problems is somewhat difficult and that area of the back isn't always included in an exam for low back pain.
Here's a Spine-health forum post on the subject:
"I have had severe back pain lately and I suspect SI problems because it is worst when there is pressure below the waist on my back side. Will sacroiliac joint problems show up in an MRI? What would be the best way to diagnose this? If an MRI would come out inconclusive anyway, I’d rather not pay for one."