The button on the controller might be stick due to dirtiness, malfunction, or broken buttons. As a first step, try unplugging the controller and plug it back in.
If that doesn't work follow the [[PowerA_ProEX_Xbox_360_Wired_Controller_Troubleshooting|troubleshooting steps]] and open the controller, clean all the dirt from the inside and make sure the button is properly connected and not broken.