Working a certain muscle will not make the fat around it melt. That is, sadly, just not the way the body works. Every body is different and a personal trainer cannot tell you for sure where the fat to fuel your workout will come from. Working a muscle can only make a muscle stronger and, often, bigger. So, if you want a smaller belly, doing exercises that will make your rectus abdominus muscles bigger might not be the solution. What you really want is to pull your belly in, but that's mainly what the transverse abdominus muscles do, and crunches do not work these muscles. To make the matter worse, sit-ups also work the hip flexors, which, when tight, pull your pelvis in an anterior tilt...and make your belly stick out!
Goal of doing sit-ups vs. What actually happens
What you want to, is to work on your postural muscles, to flatten your back and pull your belly in and the best way to do so is through isometric core exercises, balance training, and postural correction stretches.
2. They do not always work the right muscles
When doing crunches and sit-ups, you expect to be working out your abs, but the truth is that even if you feel the burn there, it might not actually be the main muscles getting a workout. Instead, you might be working out muscles that do not really need strengthening for most people since just sitting around all day is already making them quite tight: your hip flexors. Indeed, the hip flexors are muscles that bring your thighs and torso closer together and what do V sit-ups do? Bring your thighs closer to your torso!