Hi,
I don't think multiple <xsl:when>s with one <xsl

therwise> will work. I haven't used or seen multiple when conditions with single otherwise in GC3.
Have you tried something like this?
if shipment refnum value = ABC or shipment refnum value = DEF or shipment refnum value = GHI
then use template A
otherwise
use template B
I have implemented my conditions this way manytimes and it works fine for me.
Also, please check if your xpaths (eg. /notify/Blob/xml/TenderOffer/Shipment/ShipUnit/ShipUnitContent/ItemQuantity/ItemTag2) are fetching the correct values.
Let me know if you need more info.
Regards,
Satya