Hi Jlm
Presuming that your objects are lines and not smart polygons or some other object out of the change object properties dialog box, and that the two lines are on the "same plane" the one thing you can't do is use the angular dim tool on two lines that are joined. For example if two lines form a "V" and you have joined them with the join curve tool that's a problem. The program thinks your trying to dimension a line to it's self and doesn't want to snap to the other half of the "V".
Having said that most like cause of many problems such as this is that the lines are on a different plane albeit slightly. Good practice is to work in 2D mode or turn on the snap to plane tool if in 2D3D. Or for referencing points off plane in 3D, but creating object on a user plane, try using the snap to work plane tool.
Chuck