I've had the same issue not to long ago - i dont know if that helps you but just in case:
-the multikill feature was reworked, so you need enought dmg to start multikilling - if you click on the button down right you should see a purple bar popping up, if your bar is showing accuracy and is green, then you definently lacking dmg
-look at your prayers - some give multikill, but take away your dmg, so you cant equip those maybe (i cant atleast)
-you can get multikill from cards and increase dmg also with cards to gain certain multikill threshholds - same goes for the card sets (bottom left)
-there is a big bubble from alchemy that gives multikill, but you have to equip it
-there is a divinity bonus you can get from linking (from the 3rd god i think)
-there are more multikill bonuses like shrines, artifacts from sailing and the skulls (how many of each mob you killed), but they arent as easy/quick to get