Pads can get damp and start to stick. Fish skin and leather pads, as well as many synthetic pads, have a tendency to do so.
Powdered papers can quickly remedy this. Careful cleaning of the tone hole edge and pad can also make sense. Unfortunately, it doesn't always help. Light powder dust can lie over the instrument. Therefore, it should only be used in an emergency. If that doesn't help, the pad needs to be replaced.