I have had this same problem. I would lie down in the afternoon, start relaxing every part of my body with each breath, and quickly reach SP. However once I was in SP, my expectations of what I wanted to achieve were actually what were holding me back from achieving them. So I began to just enter SP with no thought on my mind. No desire. Nothing. And after a few attempts, I was able to enter a vivid dream, become very aware I was dreaming, and still keep the same mental attitude I possessed before hand: Almost completely desire-less. So try that out. Hit SP and keep your mind blank. Eventually your subconscious will take over and surround you with whatever its beholds.