Meditation & Altered States

Indeed. It does sound like you are struggling to get into deeper altered states because you are unable to let go of the physical sensations of the body. Your attention on the physical body thus stimulates it and tethers your consciousness to the physical realm, preventing you from opening up the senses required to attend to other realms of information (such as the astral). I recommend that you try not to focus on physical sensations and instead put that attention into simulating and embracing falling, swaying, driving, or drifting sensations, whilst allowing your mind to wonder about the internal world as you would casually admire and appreciate the natural world while on a boat down a peaceful stream, laying on a hill and watching the clouds and feeling the wind go by, or in the back seat of a driving car looking out the window of things passing by. Have you tried that kind of exercise before?