What to check
If this is a multi button panel, then try shorting the two screws on the suspect button.
If this is a one button panel, the button should have some 'tactile click' feel to it, if not then the button may have failed.
Corrective Action
Replace Button, or if one button panel, send to us for repair or replace 500-LS pcb
Links
https://www.entryphone.co.uk/shop/productdetail/PB92-S
https://www.entryphone.co.uk/shop/productdetail/PB92-P
https://www.entryphone.co.uk/shop/productdetail/PB-VR
https://www.entryphone.co.uk/shop/productdetail/PB-VR-P
https://www.entryphone.co.uk/shop/productdetail/PCB-500-LS