In my opinion, it is likely due to the fact that your body is more relaxed in the altered state so there is less resistance to the energy of the mantra. Bodily tension creates blockages and hinders the flow of energy so it can feel like the energy is less intense or weak because it has to flow around the blocks. It’s like putting a large rock in the middle of a stream. While the stream isn’t completely stopped from flowing, the water that does make it around the rock will flow noticeably slower, with less force.