when doing the back leg lowkick on the bag, I always wind up making initial contact with my foot instead of my shin. A sore foot and a ripped off toenail later has sent me to ask for advice on what I could be doing wrong.
You might be holding your hip back, which kills your power and range. Make sure you're rotating fully with the above tip.
Are you pointing or flexing your toes? I usually have my toes/instep in a straight line with my shin, and flexed (like trying to pick up a towel). That'll stop the toe nail from getting hit, and it will help a bit with the pain in the top of your foot. It'll still hurt though if you hit with it.
Also, look at your setup, it could be that you are telegraphing your kick and your opp is moving back enough that your foot is hitting it. Work on hiding your kick with punches.