It's likely got "stuff" in the pivot points and in the groove, as mentioned. I finally had to put a seal kit in ours because it got to be a real pain. If you do take it apart, I used silicone plumber's grease everywhere that even looked like it moves. After two years, it's starting to not close the whole way again. I think on some level, it's the nature of the beast. Another mediocre execution of a mediocre design.
"It`s not important that you know all the answers, it`s only important to know where to get all the answers" Arone Kleamyck
"...An unarmed man can only flee from evil, and evil is not overcome by fleeing from it." Col. Jeff Cooper
Sunset Creek 298 BH