Make sure they use the charged ends for the observations. To discharge a rod you need to rub it all over with your hand. Make sure whoever does this is not wearing lotion, or it will never charge up again until it is cleaned. Dish soaps will inhabit the charging process, just like lotion. Humidity can cause big problems in this activity.
Objects lose their charge very fast in humid air.
In case of high humidity the use of a hair dryer or a heater is recommended.