InnerVoyager
My guess is that there is a certain amount of electrical and heat energy contained in every wall (electrical wires inside each wall and the normal heat retained in a wall). Normally this energy will not be a barrier to passing through the wall. Sometimes, however, when OOB when a person's own energy is low or their concentration and focus are lacking (as was the case during my latest OBE) the low amount of energy inside a wall DOES act as a barrier and prevents the person from passing through.
Grey
My guess is that there is a certain amount of electrical and heat energy contained in every wall (electrical wires inside each wall and the normal heat retained in a wall). Normally this energy will not be a barrier to passing through the wall. Sometimes, however, when OOB when a person's own energy is low or their concentration and focus are lacking (as was the case during my latest OBE) the low amount of energy inside a wall DOES act as a barrier and prevents the person from passing through.
Grey