No clouds at high altitudes?

Hello. My problem is that I cannot experience overcast conditions at high altitudes. For example when cruising at 37000 feet I go to "set wx" menu and try adding cloud layers with high base eg. 39000 or 33000 feet oe whatever and setting big thickness value it doesnt create them. It is always the same. I can see the nightsky and stars clearly every time climbing pass around 14000-17000 feet. I know there are high altitude thunderstorms that cant be flown over in real life, why cant I simulate one in as2012? Everything works fine at low altitudes (below about 14000 feet). Am I doing something wrong with the settings? Using DWC, flying PMDG ngx.

Anyone?

Thanks!

Kaj Ekström