Do you happen to have a second key you could try? The key you are using could be worn too much to trip all the needed tumblers in the lock cylinder.
Or, the lock cylinder could be failing. You will likely need some professional help to try and troubleshoot and work to get this resolved.
RepairPal Certified shops can diagnose and repair your 2002 Mercedes-Benz ML320. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.