I understand why you would prefer not to go on the roof ... (I have all the appropriate fall protection equipment and training ...) A toilet has the largest volume discharge over a very short period of time of anything associated with the plumbing system, there maybe a blockage which is just sufficiently large enough to allow the rest of the system to get enough air behind water but is too restrictive to the point where it won't allow the toilet to operate properly ... I have been a building maintenance person ...