Without seeing a video of you bench pressing, it is hard to see what you are doing wrong. Here are a few common things you can do to involve your lats more in the bench press:
1. Tuck your elbows more towards your body. One of the most common mistakes I see is keeping the elbows too high so that your arms end up nearly perpendicular to the torso. This is not only unhealthy for the shoulder joint, but also makes it tough to use the lats. Therefore, keep the elbows closer to the body, though not too close that you end up doing a tricep extension.
2. Proper setup. You need to squeeze your shoulder blades together, arch your back, keep your ass on the bench, and have your feet flat on the floor. You need to keep your upper back tight; if anything is lose and wobbly, you won't be able to properly engage the lats.
3. Strengthen your lats. If they are weak, you won't be able to use them much. Balance every set of bench press you do with a set of rowing or pull-ups. This will also have the added benefit of preventing muscle imbalances and protecting you from shoulder injuries.
Finally, a simple exercise you can do to engage your lats: lie down on a bench and get into the proper setup position as in #2. Unrack the bare bar and slowly lower it to your chest and pause there. Now, instead of driving back up with your arms as normal, just flare your lats out to the side. If your lats are well developed, this alone should lift the bar a couple of inches (if not, see #3). Once you get a feel for this, try to incorporate this into the full bench press motion. The idea is that the weight stretches the lats (like compressing a spring) as you lower it down and you can then use this stretch reflex to start the drive back up by having the lats recoil and driving with the arms/chest simultaneously.