If a pH probe is not holding a calibration immediately after the calibration process is performed with freshly prepared, accurate standards, then there may be a few things wrong with it.
First, consider the age of the pH Sensor. If the barrel of the sensor is a gray-green color, it was manufactured between 2000 and 2007. After several years, any pH sensor will lose its sensitivity, or it will have stopped working altogether.
If the barrel is transparent blue, it was manufactured between 2007 and October 2012.
If the barrel is opaque blue, it was manufactured after October 2012 and is currently still shipping.
Inspect the globe-shaped tip of the sensor. If it is cracked or broken, then the pH sensor is irreparably damaged and needs to be replaced. We cannot repair a broken bulb.
Try a quick reading of the pH Sensor. Take readings in fresh buffers, NOT distilled water! If no buffers are available, try it in vinegar (~ pH 2.5-3.5) and ammonia (~ pH 10.5-11.5). If readings do not change when the sensor is placed in different solutions, the sensor is possibly defective or damaged.
Why doesn't my pH Sensor read pH 7 in distilled or deionized water?
If the pH sensor gives readings, in different buffers, that are ~13.6, then the electronics are probably dead and beyond repair.
If the pH reads 7 and does not change in different buffers, then the sensor portion is dead and beyond repair.