First: make sure the hitch is completely open before you drop the tongue on the ball.
Second: You usually need to pull the tow vehicle forward a bit to seat the hitch ball in the slight bulge in the front of the socket before the lever will drop down fully.
I leave my chocks in and the expanding chock between my duals until I am fully hitched up. This allows me to put some forward tension on the ball without the trailer trying to follow me.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10