If your mail keeps getting stolen from your mailbox you need to contact the police and contact the postal inspector.
The Postal Inspector is police for the USPS and does investigation on mail theft and anything illegal happening with the USPS.
Also consider putting up cameras that are aimed towards your mail box to catch the mail theft on camera as that can help prove who is stealing your mail.
Another thing to do is find out what time the USPS delivers your mail to your curbside box or house mailbox and then check the mail soon after that time.
I do this myself and get my mail early in the morning so the mail is not sitting in my mailbox for long.
I work from home and the mail carrier comes to my mailbox around 9:50 AM to 10:30 AM depending on how busy they are.
I then check my mail at least before 12:00 PM to remove any mail that is in their.
They also sell locking curbside mailboxes as well that the mail carrier can put your mail in and you have a key to get the mail.