I don't know if anyone else feels this way...but for atleast the last three years I have had a constant tingling sensation in my third eye that I could intensify just by focusing on it.
But every time I meditate, I can somehow release a huge part of the pressure and feel hollow exactly at the center of the eyebrows. But it still feels like I am holding a small part of the chakra from releasing itself, like I've caught a muscle or something. And whatever I try to do, I can't come around releasing this seemingly small pressure.
There is no need to worry about the specific details involved in the opening of the ajna chakra. At times it will feel as if nothing is happening, at other times it will feel as if a lot is going on and there are changes taking place. Everything happens in it's own time so there is no need to be concerned.
The important thing is to simply practice Deep Meditation and Spinal Breathing Pranayama twice a day. The opening of the ajna chakra will then happen at the right time and in the right way.
More detailed instructions for Spinal Breathing can be found on the AYP Plus website here