Two hits

[Scrapper1]

I worked a match recently in which one team's bench complained that I was not calling double hits, to the point where I issued a yellow card to the assistant for gesturing about it several times. I seem to have trouble seeing this fault, and the head coach seemed to think that it was there on every other rally.

Do you have any suggestions for recognizing the double hits? Is it a fault just because the setter tries to set the ball to one side? Adding to this is the NCAA directive this year to be slightly more lenient on ball-striking in some cases. So I'm kind of at a loss for this particular call. Any help would be appreciated.