There are various reasons for a shift in photopeak.  It is best to recognize the differences and correct the problem immediately. Most equipment is relatively stable once the desired photopeak is identified and set.  Shifts usually occur if there is a problem with the voltage or power supply to the unit or if the camera is not set precisely on the desired isotopes energy peak.  Tc^99m has a photo peak of 140kEv. Similar to tuning into a radio station, if the camera is set above or below 140kEv the reception will not be clear and pick up information (noise) from adjacent frequencies or lose information altogether.