There are several things that can affect the signal strength and interfere with advertisements from beacons. These can include beacon mounting surface, obstructions in the environment such as metal, water (this includes our bodies), and other BLE signals.
As a general rule, you want to calibrate a beacon to its environment with the appropriate settings. This includes alterations in Target Speed and Loudness. Additionally, we recommend that Beacons not intended for NFC like use be placed high enough to avoid interference from physical obstacles.