Heavy attacks not working

I'm returning to the game and attempting the duviri paradox quest and have reached the guy who gives yoy the sun and moon swords then fighting the dax. But when I reach the part of that fight where the quest is "do a heavy attack" despite trying everything to get it to work (combing then pressing heavy attaxk, spamming it etc) it just never happens so I can't progress. I've tried restarting the game and it still doesn't work. Any help? Thanks in advance