| I am not an expert, but I know how you lift does influence the results. It would be good to read up on it. If you are lifting weights and doing a few sets of 10 or 15 you are probably just building endurance and possibly burning away some fat. Your muscles are unlikely to get a lot bigger, but you should be able to go longer.
You see bodybuilders with spotters grunting and groaning and pushing to failure. I don't know if there is any new theory or not, but the old theory was you had to rip muscle a little bit to build it. You push to failure and make sure you have your protein to help the body rebuild the muscle. You will also see that bodybuilders give their muscles time off. One day they will work legs, one day they will work their upper body.
It might not hurt to consult with a personal trainer for a time or two. Have a goal in mind and have them help you build a plan that fits you. The fact you are working out is great, it is keeping you in shape and is probably helping you get more athletic, but if it isn't meeting your needs, talk to an expert.
The other bigger fad I am seeing is circuit training or cross training. The body gets used to the same old, same old routine and it may not do as much for you. It can help to switch up the exercise for your body and your interest level.  | 
