Cut, frayed, or broken cable. Bent or broken connector. Rinse electrode thoroughly with deionized water before use. Gently blot droplets from a wet glass electrode bulb with lens paper or soft tissue by contacting only the droplets to soak up excess solution. It can take many minutes for the electrode to return to normal operation.
Clothing also can produce a static charge that affects electrode response.
Gel-filled electrodes do not require filling, but should not be left in dilute water for long periods of time. Check that the reference junction is not blocked. Rinse off salt buildup with deionized water. Check that you can observe seepage of the filling solution through the junction. Always unplug the fill hole before making pH measurements, and replug it after use.