Maybe you have an unconscious strong expectation in mind when you do it.
When we try to "imagine" what breathing in energy feels like, we unconsciously create an expectation about it, and this only blows our experience.
When you do this meditation, be sure to be fully relaxed and to empty your thoughts in preparation for that. While you do it, try all you can to remain open. By "open", understand having no expectations in mind and being receptive to any kind of experience of which you might have no idea. When we create an expectation, this closes our mind to any experiences that might not exactly be the one we're expecting. Each one of us feels the energy in different ways, so the way you'll feel it might be totally different than you expected. So just remain open, willing to accept any way of feeling the energy, and just observe what you feel when you do this meditation.
Also, try not to "think" of anything, it's more important to feel the energy.
And the way Juan Liria described it in a post may also help:
You try to feel like you're physically breathing in air through your nose and exhaling through your mouth, but at the same time psychically breathing energy through your skin.
Hail Satan!