Just a thought...are you backing under the kingpin and then lowering it? Or are you backing into the kingpin, lowered so that your TV raises the 5er slightly when you back? I found that backing into the kingpin will cause the hitch to completely snap shut securely. Think about it...that is how truckers do it. I would back until the pin was about to engage, hook up my power connector and set the brake on the 5er, along with wheel chocks to hold it steady while engaging the kingpin. Hope this helps.
Lee
2001 36' Kountry Star DP