I took notice of the following during my evening DM session. When I start to sit and start saying mentally the mantra, first my attention goes to my body (to arrange it properly, relax tensioned parts etc.). Then impressions of things that happened during the day start to come up. After this, dreamlike impressions start to come up, that make my attention go away from the mantra. Further I didn't come still
Don't worry. The stillness is there, your perception of it is just being blocked by the activities of your mind. This is normal. This is why we meditate.