The parent unit is probably close to the boundaries of the operating range. Put the parent unit closer to the baby unit in a place where it has better reception. Please note that it takes about 30 seconds before the connection is re-established when you move one or both of the units.
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.