Show Variable from detail in Header

Here is my data..
<PO_Print>
<On_Vendor>
<On_PO>
</On_PO>
</On_Vendor>
<On_Vendor>
<On_PO>
</On_PO>
</On_Vendor>
</PO_Print>

My template has a page break on Vendor.
I have a variable in the "On_PO" section that I need to show in the page header and I can't get it to show up.
Right now I'm just calling the variable in my header template.......
<?xdoxslt:get_variable($_XDOCTX,‘POtext’)?>
and it doesn't work.
Thanks for the help!

Just want to know, where set<?xdoxslt:set_variable()?> is called before this get variable ?

I was told by a consultant that calling a variable in a header sections when it's being assigned in a for next loop in the body/detail section is not possible.
I have solved my problem a different way using Bursting.