Admonitions in .epub documents?

I've been trying to get admonitions working in Docbook 5 / epub output 
files without any success. All that I get in .epub files is a blank 
space where the admonition should be.

They are generated for PDF, HTML books, and CHM files without any 
problems. I'm just using the docbook.xsl stylesheet included within 
docbook-xsl-ns-1.75.2.zip.

Is there anything special I need to do? The xsl only appears to have 
settings for callouts.

Thanks,

Glenn