There may be interference from another appliance or baby monitor. Switch both units to another channel. Make sure both units are set to the same channel.
The parent unit may be too close to a window, causing it to pick up signals from other appliances or baby monitors. Move the unit away from the window.
The baby unit may be out of range of the parent unit or there may be too many walls and/or ceilings between the two units. Try a different location or reduce the distance between the two units.
The batteries may be low. In that case, replace the batteries of the baby unit or recharge the battery pack of the parent unit.