Most likely the 30amp connection in the box is worn and not allowing good contact. This generates a lot of heat and can even melt the plug. As already mentioned, warm is ok but hot is something to be concerned about. Make sure your plug on the 30amp cord is clean and bright so good contact can be made inside the receptacle.
In addition, I would get a
50amp male to 30amp female adapter and use that to plug into the pedestal on the 50amp side. That outlet is usually in much better shape and won't destroy your expensive 30amp cable and plug. I use mine all the time. There is no danger to your rig from using that adapter.
Barney