There might be some interference with other 2.4GHz devices, such as your mobile or portable computer, which are connected to your home WiFi network and are located very close to the parent or baby units.

In order to avoid such interference issues, it is always recommended to keep such devices at least 3 feet away from the monitor. In many cases it is even enough to move your mobile device about 8-10 inches away from the monitor and this should solve the problem.