Ben hit the probable iisue.
Separate the switches and try them, It should be fine.
If not, the switch is malfunctioning and needs repair.
If it works separated, then place a small bit of duct tape or electrical tape on the ends of the outer rails. This should fit it.
The cause is probably a slight imperfection in the angle the switches meet at or a slight non-level aspect of the 2 switches.
Switches need to meet the approaching track level and square or you end up with the anti derail making the switch stay one way, if both departing tracks are hitting the switch will buzz/chatter and not work.