I'm going to assume that you are getting good penetration, you just have a hard time finishing. If you are not getting in deep enough that is more important to figure out since it will make finishing a shot that much easier.
When you get in on the leg (I'm going to pretend that you shoot to YOUR right side/your opponent's left leg just to lessen confusion) there are three things that I teach my guys to do depending upon how hight or low they are.
If you are low on the leg--below the knee.
Try and cup the back of his heel with your right hand and grab his knee with your left. You are basically creating a lever and using his leg and the leverage that it can create against him. Once you have the heel and the knee try to straighten his leg out as you scoot back (more so away and to the side) and come up. Once you are up you can do several things, trips, sweeps, elevate, etc. etc.
if you are on the knee or close to it.
In my opinion this is the hardest place to be, so don't stay here long. when you are attacking the knee you have your arms pitted against his legs, and I can almost guarantee that his legs are stronger than your arms. As soon as you get here lock your hands and try to pull his knee in to you and DOWN. PIN HIS KNEE TO THE MAT. Then make sure that your knees are off the mat, gue up on your toes and drive all of your weight onto his knee. Circle to your right keeping his knee on the mat and keep looking for an opportunity to come around and cover. Your opponent will probably try and wizzar down hard. The more weight that you put on his knee/leg the harder it will be for him to circle with you. Sooner or later you'll be able to hook his letf leg and work up his body to the hips.
If you're up hign on his hips.
Drive, Drive, Drive and elevate. You also want to keep circling. Feel where the pressure is coming from and try to let your opponent dictate your finish. If you are in on a single and he is driving into you hard you will probably be able to circle around to the side. I'll elaborate more on this one later, I need to get class started. I'll try and get some video up for you as well.