There aren't any maill workers who sit inside the mailbox house. Your mother should ask some neighbors when they think the workers are there. The workers arrive, dispense the mail & leave. But many neighbors are surpisingly good at knowing when your postal has the workers available. When the workers are there, ask someone who has opened their box if you/or your mother can call in through the open box. Just say Hello, hello? & when they respond tell them your situation. The solution may be any one of those suggested: jammed mail, lock needs lubrication, lock needs to be replaced. In the meantime the worker will hand your mother her accumulated mail.

Good Luck