" What is the block? Why is your mind not developing? Why are you not getting better and better? Over many years you have been listening, reflecting and meditating on the path. You have heard and read many times the most profound secret tantric teachings, especially of Maha-anuttara Yoga Tantra. You have even done some retreat. But your mind is still the same as before. A rock that has been under the ocean for many eons is still hard, with only wetness on the outside, no wetness inside. There is no change - nothing goes inside. Your mind is like this: not having any change, any development, any realizations of the path. Nothing of what is mentioned by Buddha in the texts of sutra and tantra is being actualized in your mind.
This is all because of not having done powerful, continual purification. Your mind is blocked by all these heavy collections of the various negative karmas and root downfalls accumulated with your body, speech and mind in this life, and in beginningless past rebirths. The first thing is not having done perfect, powerful, continual purification. The second thing is not having changed your actions. Particularly those who have taken the three levels of vows, even if you do some purification, you continue to create the negative karmas by receiving the vices or root downfalls of those vows. You continue to obscure your mind. Even if you have done some purification, by not changing your actions, you continually obscure your mind."
says Lama Zopa Rinpoche
in Purifying with the Four Powers