well i don't know if it may help you but this reminds of a similiar problem where the shadows tended to show up or not depending on the angle and position of the camera ....
from the online manual "SHADOW" : "Shadows are only cast by entities when they are potentially visible, i.e. when their visual bounding box touches the view frustum and is - in case of a BSP level - not hidden behind a wall. If you want shadows to be cast by offscreen or hiding model entities, increase their visual bounding boxes by placing dummy vertices in MED."
maybe if you add some vertices to the entity which emits the light to create a bigger visual bounding box ...
but just a guess