you're using plus/minus tolerances, that's why model is underconstrained.
Here's an explanation of how the software works when you use auto-dimension on turned parts wth plus/minus tolerances:
• The planar face selected as the reference feature defines the origin for all locating dimensions.
• Each diameter is dimensioned for size and shown as a linear dimension in a plane parallel to the part axis.
• When the Tolerance type is set to Plus and Minus, no dimensions or tolerances are added to control (coaxially) the diameters to one another. As such, each of the diameters is under constrained. This can be visualized by using Show Tolerance Status (DimXpert toolbar)
Thanks Alex, that is good to know.