If the turn meter doesn't get depleted, probably you're lacking accuracy. But failing to crit, if you have over 70 percent crit rate, shouldn't be possible... especially since she's void so it can't be a bad affinity matchup. 

Thanks, I'll try getting her accuracy up. 

The basic rule of thumb for accuracy is you need about 10 * the stage number in order to apply the debuff in the large majority of cases. So for stage 13 in dungeons, you need at least 130, for stage 20 you need 200+. 

She had around 160 accuracy and I was struggling with Fire Knight 15 so I thought it was weird that she hit pretty bad and she couldnʼt deplete the turn meter. I switched some gear around and got her accuracy to 200, I'll test it to see how she does. 

On Fire Knight, of course nothing will work as long as the shield is up - you meant with the shield down, right? 160 accuracy should work most of the time on FK 15, it might occasionally fail but not very often. 

Yes with the shield down, she never uses heartseeker when it's up. I ran it three times and it doesn't fail anymore, thank you!