I am certain that this issue has been brought up many times, but as long as Visio keeps having these problems, this issue should be brought up again and again.
So, the problem with Visio is that it tries to always snap to something else where user would like to. When I have everything lined up according to the grid, and the diagram consists of straight horizontal and vertical lines in perfect harmony, Visio wants to snap just a little bit off the grid and make inclined lines. Sometimes it is just impossible to draw a straight line to a desired location, eventhough it is the best place according to any rule; so you have to first draw the line to somewhere else and then move and stretch it to actual location. However, if I would like to get some lines a little bit off the grid, of course then Visio strongly snaps to grid or shapes or anything except to the point that I would like to. It is just like it wants to make drawing as difficult and painful as possible.
This has always been a problem with Visio but it seems that Visio 2016 is worse than ever, in every aspect.