Have you been trying the rope or roll out method? These two are very similar and common practices for those having a hard time getting their astral body out of their physical one.
I also recommend trying the following; Pick a physical place that you are very familiar with. As you meditate go there in your mind. See, hear, smell, taste, and feel everything about this one place. See what surrounds you, hear the other beings that may be there, smell the common smells associated with the place, feel yourself there. This is somewhat a trick of the mind, but for some reason it seems to help some spontaneously AP to the place they focus on.