Hank Gaylord

So, I got my truck in September of 2017, and made sure that the truck went into 4Lo. It did. Since then however, I've tried to put the thing into 4Lo and it just wont go. It goes into 4Hi perfectly fine, but whenever i go into neutral, stop, and flip the switch, it just wont go. I did get the transmission serviced and all the fluids changed when I got the truck, but idk what it could be. All it does is beep at me and flash the 4Lo light. Has anyone had similar troubles? Any suggestions? Thanks so much!