Could you have activated the Zero Radiation Function?
Activating the Zero Radiation Function makes the baby units stop transmitting signals to the parent unit in standby mode. Radiation = Zero. In standby mode (when the microphone in baby unit is not detecting sound), you will not see temperature or battery status from the baby unit that has activated ZRF. Although, this information will be displayed when the baby is making sound because the information then will be transmitted together with sound signal.
NOTE: When the ZRF is activated, the out-of-range alarm will be deactivated. For this reason, only use the ZRF when you know that the units are within range.