what have you done to troubleshoot?
more info is needed since no one here can see the phone and how its setup.
Are you rooted?
How long have you had the phone?
Do you have a exchange account that needs its own set of rules?
Have you had this issue since day one?
Do you have the 4.3 update if its avail on your carrier?
have you used the 4 digit pin unlock option?
Are you using a 3rd party app that takes over the screen lock and/orr home screen (widgetlocker type programs?)