I am trying to mate two parts in an assembly. One is designed as a split ring and would slide over and stay in place because of the elasticity of the material. It will not mate these items.
Welcome to the forum. If you have the part selected and highlighted in the tree when activating the Mate function, Lock will be the mate type selected by default. If that's the case, right-click on the part name in the Mate Selections box and choose "Clear Selections". That will allow you to choose a different mate type.
The Lock mate would (and should) be the only available option if you are selecting complete parts to mate. You need to select suitable features within parts (holes, faces, planes, axes, etc) to enable other mate options.
Another potential cause may be the surface type. Often a revolved part generates toroidal or conical surfaces. SolidWorks 2014 addresses some of the conical section mate issues, but if you are using 2013, you may need to mate via planes and sketch geometry.
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