Problem for drawing Sphere

My system is a 3D scene for render.Now i want to add much cloud to the sky.i don’t use the rexture for my cloud,but simple model.
My method is using hundreds of metaball to simulate a block of cloud.And the shade is simple.
My problem occur in rendering in perspective projection.first all of metaballs are drawing somewhere shaping a block of cloud.But after some keydown to render,only one metaball is displayed and others are dispeared.
Who can tell me the reason for this boring problem?tks a lot!!!

You really have to be more specific (code perhaps) if you expect anyone to help you…