Often users complain that join polylines tool is not working as it should be. Here is a simple tip to get your "Join Polylines" tool working
keep your VERTEX snap mode on and then hover the cursor on the end point of first polyline. A red cross will appear on the end point, click on that point. Now, the next line you draw will start from the end point of the first line. To join the polylines, it is necessary that their end points meet with each other; and this can be achieved easily by drawing the polylines in vertex snap mode.