Three.js shadows not working properly -

i trying learn shadows in three.js , found this nice example in jsfiddle. not able understand why, when lower y of light 65, is:

light.position.set( 20, 65, 0 ); 

the shadow disappears completely. meanwhile, above 70 fine , shadow cast. always, missing obvious, cannot see can preventing light making shadow.

you can try:

light.shadowcameravisible = true; 

to see position , direction of light on screen understand changing.

hope helps :)