It has been my experience that if there is more than a few degrees difference in angle between truck and trailer, the ball and coupler can indeed resist coupling, and if the coupler you are using is like the BULLDOG-CHANNEL OR A-FRAME STYLE COUPLER.. http://www.ontheball.com/trailer_couplers.htm
the one located about half way down the page, I can see how it would bind if not aligned.
Like you, I have built ramps to drive onto to minimize the angle. I also keep the latch and coupler mechanisms lubricated (WD40) to help ease the situation. How difficult is it to couple and un-couple when the truck and trailer are on level ground? If the sliding mechanism resists everytime, no matter what the circumstance, I would definitly get it looked at, but if this problem is just limited when angles are involved, I would suggest modifing where the trailer is parked, or build some work ramps for your truck.
Anyone out there with a T160 experiancing the same issue?
Good luck tapelgan, I hope this helps.