The original Volvo wiper arm slowly bends in use and eventually doesn't put enough pressure on the blade to keep in contact with the window. Poor engineering work, pure and simple. You can rebend the arm which I did once with some success, but a few months later it had bent itself back. Then I bought a Scan Tech replacement arm which has lasted at least four years now.

Volvo seems to have trouble designing some of it's cast metal parts. The outer door handle sections of 240 door handles are also prone to changing shape all by themselves!