Commode flush does not completely empty contents in bowl. Am reasonably sure drain is clear, and have checked small hole in front bottom of bowl. It is completely clear. I suspect leaves or other matter in vent, but I'm not sure how to remove the stoppage or otherwise free the vent. Can I use a "snake to just break up the stoppage, or must it be somehow physically removed from the vent?

Have you checked to make sure that the flapper is not closing too soon?

I had the same problem once, and it was due to the flapper closing before the tank had even drained 50%.