I figured out the problem. Turns out, because I tend to be ruthlessly efficient at keeping only my best and favourite weapons in my arsenal at any given time, I didn't have a weapon crap enough to actually let me rack up a multiplier - one hit kills force you to peak at about x2.5 when I desperately needed x3.
In the end I had to strip ALL of the mods (apart from Fury) off'f my Venka just so that I could hit something more than twice before it died. Good times!
It was worth it, though. For the first time since they came out I now have a Riven for a weapon that I actually want to use, and it's doubly great because I also learned that, unlike Syndicate mods, Rivens work on variants of a weapon - Boltor is the same as Boltor Prime and Telos Boltor, so that's kinda great. Time to start spending some Forma.
Shame that the other Riven I opened was for a Gazal. Machetes..... Free to a good.not so good home, if anyone wants it?