Sparky,
The Lee Loader requires that you use brass that was fired in the gun you are reloading for, or new brass. If the cases were previously fired in another gun, they may not fit the chamber of the gun you are using.
Another likely cause of difficult chambering is if the crimp is too firmly applied, it will make the case mouth bulge out and and prevent full chambering and or closing of the action.
HTH
John