Thanks for the help. I had a few minutes to look closely at the tongue and see what people mean by the "coupler being pushed into the tongue." This, indeed, seems to be the issue, as the coupler is retracted into the tongue.
I'm not sure why this has just started to be a problem.
I think that the reverse light signal is working properly, as I'm able to back up the trailer with the Xterra attached. I'm wondering if I need to do as JRW suggested - - - chock the wheels and roll forward slightly before detaching the boat to pull the coupler out.
I still wonder why the coupler is getting hung up, if that is indeed the case. It never did this before 1 month ago.
186,000 miles per second - - - not just a good idea, it's the law.