Perhaps I missing something glaringly obvious here so could do with an extra pair of eyes. I have simplified my example to make it easier to demonstrate.

Here is my XSLT:

    <xsl:param name="currentPage"/>
    <xsl:variable name="title" select="/macro/Title" />
    <xsl:variable name="description" select="/macro/Description" />
    <xsl:variable name="source" select="/macro/Source" />
    <xsl:variable name="thumbWidth" select="/macro/ThumbWidth" />
    <xsl:variable name="thumbHeight" select="/macro/ThumbHeight" />

    <xsl:template match="/">
        <ul>
            <li>
                <xsl:value-of select="$title" />
            </li>
            <li>
                <xsl:value-of select="$description" />
            </li>
            <li>
                <xsl:value-of select="$source" />
            </li>
            <li>
                <xsl:value-of select="$thumbWidth" />
            </li>
            <li>
                <xsl:value-of select="$thumbHeight" />
            </li>
        </ul>
    </xsl:template>

Now for some reason all values are output except for $source. The datatype in the macro is a mediapicker however with debug/trace enabled on my page the macro is getting the required values so why is it not present when I try to use it or output its value?

Stack Trace:

renderMacro

Rendering started (macro: Image  Gallery, type: 1, cacheRate: 0)

umbracoMacro

Xslt node adding search start (Title,'Sample Image Gallery')

umbracoMacro

Xslt node adding search start (ThumbHeight,'100')

umbracoMacro

Xslt node adding search start (Source,'1059')

umbracoMacro

Xslt node adding search start (Description,'This is a sample description of 
the image gallery')

umbracoMacro

Xslt node adding search start (ThumbWidth,'100')