Trige,
I had a similar problem with one of my wiper arms. The problem can be that the splines on the shafts get partially filled with metal from the arms and since the splines are so shallow, they tend to lose their ability to engage and hold the arm in place. I resolved the issue by vigorously cleaning the shaft splines with a wire brush. It didn't appear to have changed anything but after reinstalling the arm and torqueing to spec, the arm has worked with no problem for over a year. You may also need to clean (or replace) the arm to get it to grip properly.
Steve.