Disappeared shadows- need help

I have some mesh,which generates in runtime. and i have noticed some strange things with shadows(casted by directional light),when that generated mesh is present in scene:

• i can see shadows only if directional light have rotation 0(+- 10 degrees),0,0.if X rotation value is near +- 10 degrees, shadows cutting partially

• also,i can see shadows casting if object located in far-far-avay from that mesh.

• also, if i will remove my runtime-generated mesh or just i have disabled that mesh renderer, shadows comes back!

• anyway, i can see shadows from spotlight and point light always

generated mesh have no tangents, but even if he have them(i have tested that!), i can't see any shadows when my script-generated mesh is rendered. i have tried soft and hard shadows, tried to change bias value and shadow casting range. nothing helps, but, maybe,you can help me?

hm...one more strange thing have detected - if my mesh will be rendered,with some material, which uses alpha-channel transparency, shadows comes back to scene,but they are not casting to my mesh,until any directional light is active