DB Downfield Just Stands There Instead of Trying To Tackle

I didn’t look through the form before I posted this, so if it’s been discussed already, I apologize. 
   Every once in a while, when playing Online H2H, my opponent will break for a big gain doen the sideline, and my DB downfield will simply stand there, with a WR in his face, making no effort to shed a block or make a tackle. Am I the only one experiencing this?