I'm sure you've seen it happen, just like I have, a zillion times: Your player has the ball, is dribbling upfield and all of a sudden they get knocked over or pushed aside and the ref does nothing! And then, a few minutes later, your player commits a similar offense and the ref blows the whistle. What gives??
Well, let's assume it's not bad officiating (which it certainly could be, especially in youth soccer games). Instead, let's give the ref the benefit of the doubt and assume he or she is using the soccer concept of "advantage."
In soccer, referees don't just look at a foul in isolation. They look at what happens immediately after the foul, as well. If the "victim's" team retains the ball after the foul and is still on the attack (if it still has the "advantage," in other words), the ref won't (or shouldn't) stop play. They are supposed to let play continue so the attacking team doesn't lose a chance just because the opponent commits a foul.
If the victim's team retains the ball for a few seconds and then loses it, the ref can still call the foul and bring the ball back for a restart. If the team retains the ball for a while, then the foul just sort of goes away and is never called. That lag time is very subjective and varies by ref.
Now, if the "offender's" team gets the ball right after the foul, the ref will (or should) stop play and the victim's team will restart play.
And if a foul is really, really egregious, the ref usually calls it right away no matter what.
There are concepts like "advantage" in more traditional American sports. In basketball, a player can still score after being fouled if they are in the process of shooting when the foul is committed. In football, if a penalty is committed during a play, that play continues and a team can decline the penalty if the play on which the infraction occurred gave them a better situation than accepting the penalty would.
So, knowing this won't make it any easier to see your player knocked down and the offender go scott-free, but at least you'll understand what's going on.