Long s2ram don't work, short one works

I have hp Probook 4515s with opensuse(11.2).(radeon hd 3200)

I can suspend it to ram and it wake up almost perfect(screen brightness change, but it don’t matter). But when computer is suspended long time,(I don’t know precisely, maybe 2h) and I try to wake it up, screen stay black, nothing works, even caps lock?