Cannot add an anchor point


I am working on a walk cycle. This is my first project where I use the pen tool and the subselection tool often, but I have a problem. I made an open path and only straight lines.
This is my “template” of the leg so I can edit it in each frame.

Now I want to add an anchor point on my path. The help and several tuts on the internet say that you just have to place your pen pointer on the path and click. This doesn’t work
for me and there doesn’t appear a plus sign at my pointer if I’m on the path. If I curve the path, it does work.

Does anybody have an idea how to make it work with straight lines as well?