Tension is the problem. After unhooking WD bars (not chains) I know that with my foot on the brake and TT chocked I put the vehicle into neutral while DW raises the hitch. As the tongue of the TT and rear of my TV come up I slowly take my foot off the break (still in neutral) and if I am level I usually pop off. If I am slightly up/down hill is put into fwd or rev (foot on break) and the take foot off break. Sometimes if it is a strange angle it may take a few tries to have it break free.
In this venture a power tongue jack is your friend!