Multi Thread Java, but only one thread working

My Java Threads does not work independently, how to fix it? This is the initial main:

Mechanics mechanics = new Mechanics(busShop, "Mechanic 1");
Mechanics mechanics2 = new Mechanics(busShop, "Mechanic 2");

Thread thMechanic = new Thread(mechanics);
Thread thMehanic2 = new Thread(mechanics2);

thMechanic.start();
thMehanic2.start();

No problem so far, work as expected, so the mechanics do this:

public void run() {
    fixEngine();
}

private void fixEngine() {
    while (true) {
        busShop.FixEngine(MechanicsName);
    }
}

As seen here it works to fix engine forever, Inside the fix engine function:

public static List<Bus> ListBusEngineFix = new LinkedList();

 public void FixEngine(String mechanicsName) {
        Bus bus;

        synchronized (ListBusEngineFix) {
            System.out.println("Working Thread: " + Thread.currentThread().getName());
            while (ListBusEngineFix.size() == 0) {
                try {
                    ListBusEngineFix.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
        //Wait for notify if empty

        try {
            Thread.sleep(10000);

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        //IF EACH THREAD RUNS INDEPENDENTLY IT WILL BT ONLY LESS THAN 20 SECONDS
        //BUT THIS CASE, MY PROGRAM WAIT FOR PREVIOUS THREAD, WHICH CAUSE MORE THAN 20 SECONDS.
}

And for the result:

Working Thread: Thread-6
Working Thread: Thread-7

Both Thread runs as expected

First 10 seconds:

Bus: Bus 1by Mechanic 1

Another 10 seconds:

Bus: Bus 2by Mechanic 1

My suspicion would be Mechanic 1 (Thread) lock itself so that Mechanic 2 (Thread) cannot help it, as the Mechanic 1 do all the task. Anyway, how to fix it?

Note: I only use synchronized in the early for wait (for notification), If I don't put it will have network monitoring exception.

EDIT: Added LinkedList and Notify for clarification, all of the List provided is Linked List. an Class.

EDIT 2: Ok I have fixed into like this:

synchronized (ListBusDepotWaiting) {
    while (ListBusDepotWaiting.size() == 0) {
        try {
            ListBusDepotWaiting.wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

//Engine not yet fixed
//Notify the Mechanics to fix car
if (ListBusEngineFix.size() < 2) {
    //Means that mechanics is empty
    bus = (Bus) ((LinkedList<?>) ListBusDepotWaiting).poll();
    ((LinkedList<Bus>) ListBusEngineFix).offer(bus);
    if (ListBusEngineFix.size() == 1) {
        synchronized (ListBusEngineFix) {
            ListBusEngineFix.notify();
        }
    }

}

Now they can call and notify now, but the original problem still exists, which is unable to give work to Mechanic2

The wait and notify should be performed on the same monotor object.

In your code, you are waiting on

ListBusEngineFix.wait();

while notify by

BusShop.ListBusDepotWaiting.notify();

So the waiting thread will be blocked.