Nice start, and your doing the right thing by taking on individual body parts. I recommend using layers, and start from the rib cage. Try to get a better understanding of the skeletal structure before adding muscle, and be aware of the proportions.
Having a fixed head size as a measurement will help you get things better placed. Two heads high down should bring you to the nipples, 3rd to the belly button, 4th to the groin. The mass towards the medial portion of the pecs should be towards the outside which would be the pectoralis Minor. Understanding its Origin and insertion will lead you to know where this mass should be. Again by studying the skeleton first, your know this.
Look at every muscles origin and insertion, and your not only get the placement right providing your have the skeleton in proportion, but your better understand why the muscles provide the forms they do collectively, such as the three portions of the pecs, and things like what effect does the Manubrium have to the muscular striations of the pecs.
For the back, it looks like the Teres Major is originating from the thoracic region of the spine, where as this could be the Rhomboid connecting to the wrong vertebrae. The Scapula seems to have lost its presence, while there is the spine of the scapula, there is no medial border. The scapula should finnish lower than directly in line with the arm pit. The 7th vertebrae, check its location, add it, then build the Trapezius around this, rather than having such a large open space.
As you go your understand that the bony land marks are so essential that it can be a very hit and miss affair as to why one day it looks so right, and on another sculpt it looks so wrong..
Keep going at it, and id recommend looking at the Skeleton closely, then your know why knowing things like where the 5th rib is will help to place certain muscles.