Solid objects behind behind transparent ones

I've looked everywhere for a definitive answer on this but nothing seems to actually solve this issue for me.

Say I have an opaque sphere inside a slightly larger semi-transparent one, I'm finding that no matter what combination of ZWrite, Cull and ZTest I use the sphere vanishes when the box is viewed from some angles and re-appears when viewed from others. Is there a way to have the inner sphere consistently rendered using the Unity shader pipeline or is this an inherent limitation?

I'm using the forward render path.