shadows don´t work anymore

Hey mates,

i have a shadow problem in the android mode (iOS mode the same). I don´t have shadows...if i switch to Windows mode, the shadows are working. But the problem are not on all my computer´s! Only on my Windows 7 PC´s. But why? On my iMac and Windows XP notebook it´s work fine! This bug comes with the last versions of Unity. Earlyer it works fine. If i install the older Unity 4.3.4, my shadows work perfekt! What can i do? I searched so long for a solution.

best regards
Basti