Several factors can be affecting this:
you might not be lifting enough weight or lifting properly
you might not be lifting often enough or too often. You should never work the same muscle group two days in a row. Building muscle is essentially because you tear muscle fibers when you lift, and then the body repairs those tears. You should always give a muscle group at least 24 hours to work on the repairs. Otherwise, you will not build muscle at a good rate. The abs are the only exception to this rule.
You might not be eating enough protein. Protein builds lean muscle, and your body doesn't store it the way it stores fat and carbs. Make sure you eat plenty of lean protein sources every day.
You might have fat storage over the muscle, which makes it difficult to see. If a person does crunches every day, they might tone their abs really well, but if they have a lot of stored fat in their abdomen, you won't be able to see the "six pack"
Make sure you give yourself time, give your body time to rest, and work the muscle to the point of fatigue--where you cannot do another rep with proper form. If you aren't sure about proper form, get a video or a book. Stretch after your weight training sessions to keep the muscles elongated and strong.
Hope this helps you, and kudos for wanting to take care of your body!
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Lori
