Sometimes you cannot get a complete loop if you run into any vertex that has a valence of less then or greater then 4. valence is the number of edges flowing from a vert. If it's less then or greater then four the loop algorithm does not know which edge to follow out of the vertex.
This happens when you hit a tri or an ngon and is one of the reasons tri's and ngons are frowned upon.
In a sphere for instance trying to select along a line a longitude the loop will stop at the pole.
As for partial selection you can select the starting edge and double click the ending edge. Sometimes this picks the opposite section of the section. So try this trick - single click the start edge, single click the end edge, and then double click the edge just inside the ending edge. This will usually pick the section you want.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 12-10-2012 at 08:51 PM.