Supermarket simulator C

I have a problem with the following code. The exercise tries to simulate a row that has random number of people passing through three boxes, when a box is empty an element is removed from the original row and goes to box C1, C2 or C3 and once there after a delay the value of the number that is in any of those three boxes decreases; when that value is zero, it is removed from the box and remains empty so that another element of the original row enters the other. The problem is that when I run, the boxes show me garbage and according to me that should not happen.

void caja(FILA *F)
{
    FILA C1, C2, C3; crearFila(&C1); crearFila(&C2); crearFila(&C3);
    TipoDato a, b, c;
    do
    {
        x=dequeue(F);
        Sleep(2000);
        if (filaVacia(C1))
        {
            enqueue(&C1, x);

            if (a==0)
            {
                dequeue(&C1);
            }
        }
        else if (filaVacia(C2))
        {
            enqueue(&C2, x);
            if (b==0)
            {
                dequeue(&C2);
            }
        }
        else if (filaVacia(C3))
        {
            enqueue(&C3, x);
            if (c==0)
            {
                dequeue(&C3);
            }
        }
        system("cls");
        printf("\nFila: \n");
        imprime(*F);
        printf("\nCaja 1: \n");
        imprime(C1);
        printf("\nCaja 2: \n");
        imprime(C2);
        printf("\nCaja 3: \n");
        imprime(C3);
        if (!filaVacia(C1))
        {
            a=dequeue(&C1); a--; enqueue(&C1, a);
        }
        if (!filaVacia(C2))
        {
            b=dequeue(&C2); b--; enqueue(&C2, b);
        }
        if (!filaVacia(C3))
        {
            c=dequeue(&C3); c--; enqueue(&C3, c);
        }
    }while(!filaVacia(*F));
    printf("\n");
}