One possibility: If your shoulders or arms are sore, your posture is wrong. There are too many muscles involved in other places, and even if you exert your strength in other places, you will usurp the role of the master. The more isolated the exercise place, the better, and the less other places participate.

Another possibility: the posture is fine, but the weight is not exciting enough. The muscles have already dealt with the existing weight, and naturally they won't grow without too much pressure.

Every exercise should be exhausted, and the longer the exercise distance, the better.