Are you fit but relatively new to weightlifting or have just been winging it on your own without instruction? It may be time to review the building blocks of bulking up.
To begin with, are you doing the right number of reps and sets to reach your goals? If you want to build muscles as big as your body type will permit, do less than 8 reps per set. If your goal is tone and balance rather than bulk, do 8-15 reps per set. More reps than that don't usually add any benefit, and will likely leave you sore and burned out. As for how much to heft, you should select a weight that's heavy enough so your final repetition is tough, but not so hard that you have to compromise proper technique. When you are looking to build muscle lift enough weight until you exercise "to failure," which means that your last rep is so tough that you can't do one more (at least not during that workout!).
When the maximum number of reps you want, whether it's 8 or 15, feels easy, increase the amount of weight by the smallest increment possible and start with fewer reps. When you step up to a higher weight, allow yourself some extra rest time between sets.
Here's another tip: Keep a separate strength log of how much weight you lift for each exercise, along with your current reps and sets. This saves you time of experimenting to get to the right weight each time you go to the gym.