Mordekaiser not benefiting from spellvamp.

I've played alot of Mordekaiser recently (enough to get rank 5 with champion mastery in the last month) so I know about how much health I get back with using siphon of destruction on a complete minion wave with both revolver and wota and IT IS NOT WORKING AT ALL. I use siphon on a minion wave with revolver and still take almost my full dmg (60 health dmg from that.) With wota I almost take full cost still. My mace of spades is no better. I made sure to have missing health when I used it and NONE OF THE 3 ATTACKS SPELLVAMPED AT ALL. Is this a bug or was this intentional?