We recently had a question as to what kinds of thing can block Bluetooth signals and the relative properties of say windows vs walls. Apple has done a great job of listing some types of barriers and their relative interference potential. In the above case, glass is less of a problem than bricks, plaster and concrete.
One thing Apple don’t mention is water. Bluetooth is severely disrupted by water. As humans are mainly made up of water, people moving about a room can cause the received signal strength (RSSI) to fluctuate.
It’s not just barriers that can be a problem. The Apple article also covers interference from other wireless devices.