If there is something caught in the toilet, it can pass water, but solids get caught up on it. Things like a pen, pencil, toy, tampon, etc. can get caught in the trapway, but not let solids pass by, especially when augmented with a wad of paper. Often, that sort of thing can be caught and removed with a closet auger, but not always. If other fixtures are backing up, then there may be a clog further down the line. If the pipes are not sloped properly, have a dip (belly) or possibly broken or with roots growing in them, then that needs to be address. The toilets themselves may not be the greatest performers. On some, it is fairly easy to see the path...if it has sharp bends in it, it is more prone for clogs. The use of offset flanges can create problems as well. So, you may want to pull the toilets and check carefully. To assess the overall health of the drain system, you may want to consider having a plumber run a camera down the line and give you a copy of the video, if you decide to have someone else bid on any work or offer another opinion.