Maybe a stupid question, but have you tried some WD40 using the little "straw" / tube thing and spraying into the door? I've had issues like that with other cars due to the silicone grease drying out or whatever on the closer-bar / track thing... BUT, WD40 is petroleum based, and you may be better off with some silicone spray with a long straight tube if you can find some. If not, some WD40 might be a cheap quick fix.