sitting requirements for objects

I am curious to know why you can sit on some objects but not others. I have created simple objects, some are "sittable" some not, for no apparent reason. In fact, I have built an object, sat on it, left and come back, and can no longer sit on it. Is there a list of parameters somewhere? There is so much complex furniture out there, and I can't even make a simple bench to sit on. Getting a bit frustrated. Any help?