I would start by eliminating the easiest problems first. There is a lot of good input in this thread. The post by Tadd68 is a possibility, and a good place to start. Next would be the roof vent. I would stand back and observe it for a while, bees have been known to nest in vents. If your water pressure is low, it could cause a problem activating the flush kit properly. Do you have copper or galvanized supply lines? If none of these work, pull the toilet and run a snake thru it. Do you have plastic or cast waste lines? Cast is bad about rusting and collapsing. If none of these work, I hate to say it, time to call a plumber. Find one that has a camera system they can run thru the pipes, and they can see any collapses or reason for the clog. Also check your outside clean-outs. A few years ago, we had a problem with ours, the mower hay chopped off the square top part, leaving a hole. Snakes, chipmunks and who knows what else got into the drain and caused it, took a while to rod it out Good luck, hope you get it fixed.