Could be stopped down the line, Take a 5 Gal bucket of water and dump inbowl real fast and see what it does, if it goes down ok try another one.. In the tank make sure i water is up on the the mark in the tank, make sure tank ball is not leaking, check the rim in toilet with a mirrow and see if holes are stopped up, if so take coat hanger and bend it si you can clean out holes. then flush with paper and see how it does,vent could be closed, but that would be hard for me to say without seein the job, If it dont work call a lic. Plumber and let him checkit out, you also could have something in bowl, might have to take it up and see, if so turn bowl up side dow and run hose in it, also you could take mirrow and look up so far before you take up bowl let me know.
Art retired plbg