today I was trying to run about 13 cars. Four of the cars are knuckel couplers, shorter passenger cars. Two are transition cars the rest are rapido. I was trying to see how many cars my mikado could pull but the fourth car with knuckel couplers keep uncoupling. At first I thought because it was the last car it was uncoupling because of no weight behind it. So then I added the other transition car behind the 4th knuckel car and two more box cars behind that. That did not help. Last I just ran the passenger cars and the same thing happened. Why is just the one car's coupler in this new set not working. It is an althern set and all the couplers are at the same height.