Make sure water is up to mark in tank about 1″ below overflow tube
Check flapper and see if it stays up long enough to leave out water if not ajust it.
Check the flushing rim with mirrow and clean out holes with bent coat hanger
if you have jet at bottom of bowl clean it out.
Check and see if flapper leaks put egg dye in tank and see if it comes in bowl do not flush
Make sure there is nothing in hole between tank and bowl, thats all you can do except buy a new bowl if this does not work, let me know.
Art retired plbg