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All babies develop differently, some take longer to reach certain milestones than others. If your baby isn't sitting it probably means that he or she does not have enough core (abdoninal and back) strength. Give your baby lots of tummy time, that's what helps develop those muscles the best when they're young. You should also mention it to your baby's pediatrician, just so the doctor knows what's going on.
I did something we liked to call "sit em ups" with my daughter. When she was flat on her back on the floor we would hold both her hands and pull her into a sitting position for a few seconds, and then we would let her lay back down. She loved it, and it was good exercise for those muscles.