Seeing through 2 layers of glass...

[MoGrafik]

Hey everyone, I'm having some problems on a little scene I'm working on. I've made 2 glass materials, one regular glass, the other frosted.
The frosted one seems to be looking ok when rendered with the camera in the same room. However when I pull the camera back and look through a pane of regular glass, the frosted renders black. Does anyone have any idea what's going on here?
Any help would be greatly appreciated!
I've attached a could of images:

<a href="http://www.flickr.com/photos/110106132@N02/11116963006/" title="frosted only by thomhaig1, on Flickr"><img src="http://farm6.staticflickr.com/5543/11116963006_c0b3f9f157.jpg" width="500" height="281" alt="frosted only"></a>

<a href="http://www.flickr.com/photos/110106132@N02/11116925085/" title="frosted through glass by thomhaig1, on Flickr"><img src="http://farm6.staticflickr.com/5499/11116925085_85f15f56df.jpg" width="500" height="281" alt="frosted through glass"></a>

[MoGrafik]

aha - sorry I experiemented a little. Raising the specular depth in the render setting did the trick!
Sorry for the unnecessary posting!
Mod please delete, or keep open if you think someone might find this useful!

[MoGrafik]

just run into the same problem for a second time, and this time increasing specular depth didn't fix it. My glass polygon objects didn't have phong tags, adding them (with very low phong angle) fixed it.
Just thought this could be useful to someone.