No trailer power when connected to the truck hitch. Why?

Yes, the truck was running while I tried to use the power jack but no power to the jack. I also checked all the breakers and fuses inside the trailer and all were good. I really cant remember if the slides would work with the truck connected and running. I think they did but this time they didn't. I'll have to look for that button you speak of. You say its on the trailer under it? Thanks again.