There are several serious problems with the theory of astral projection, as pointed out by Susan Blackmore [Bla82]. The first is that many OBEs simply do not fit well into the astral projection framework. Celia Green [Gre68a] has collected many cases in which the person describes no astral body, indeed no other body at all. Also very few people actually report any cord, let alone the traditional silver cord.
Of course this type of experience can be fitted in by saying that the experient's astral vision was clouded, or the astral body or cord too fine to be seen, but these methods of attempting to account for actual experience begin to weaken the theory. Blackmore criticizes the complexity of the theory of astral projection as it tries to account for new facts. And this relates to the second problem, its 'stretchability.' In her opinion the theory is so complicated and flexible that almost anything can be stretched to fit it and it makes hard to draw definite predictions from the theory. If you don't see the features you should, your astral vision is not clear enough, or memory was not passed on from higher levels. If you fail to make yourself visible to someone else then not enough etheric matter was involved and so on. In this way the 'theory' is in danger of explaining everything and nothing. Furthermore, any theory which is untestable is useless in scientific terms.
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