Hi Jan - I've been very aware at times of a blockage or knot at the mastaka granthi. Located at the base of the skull, where the skull turns outward and there's a smallish bump. (Is that what you're talking about?) I've read and also have discussed with energy workers this energy center. It's been described to me as a 'pool' which can collect energy and trap it, so to speak. When the energy is trapped or 'knotted up' there, I experience an absence of energy in the upper chakras. The most effective thing that I've done to release it is to get some energy work done. The thing that works best for me is cranio-sacral work, but others respond to acupuncture, acupressure, and a good ol' neck adjustment. I also try to focus on the area that feels knotted when doing spinal breathing pranayama. I don't make a big deal of it, but on the in-breath, just wave to the spot as you pass by it.
The trick for me is not so much in getting it open, but keeping it open, as it has the tendency to draw the energy in, then horde it there. I'll be interested to hear what others have to say about this.