Measuring sit bones--is my yoga mat defective?

[OakLeaf]

Try a thicker mat, or folding the one you have in fourths. It's got to be able to compress enough to leave dents in the paper.

Set it on a hard surface - a flat stepstool, a hard chair, or an uncarpeted stair.

Before you take the impression, sit on the stool or whatever surface you plan to use without the mat, and wiggle your spine and pelvis around until you can feel your sitbones pressing against the surface. Then use that body knowledge to stick your sitbones out when you do take the impression.

HTH.