Laptop will not charge in cart, but will charge with external adapter.

We have a Dell cart that holds 30 Dell Latitude 3340's. Specific laptops have started to not charge while docked in cart. They will charge with external AC adapters. The path is the following: Power goes to cart, AC adapters are plugged into outlet behind laptop, AC adapter plugs into a female-to-male adapter that is mounted to a try, laptop slides into tray to charge. I have tried changing the battery, changing AC adapter, changing tray. Nothing seems to get the laptop to charge via the dock. Other laptops charge in the same tray.