In cases like this I troubleshoot the water closet in question with the following steps:
1) Check to see if any drop in bowl cleaners were used recently.These will break up and enter the siphon jet through the flush valve.That will in turn clog the siphon jet and rim jets.
2) Using a coat hanger ,Clean out the siphon jet. This will break up the obstruction.You can “feel” if there is something up there. Try your best to break it up and flush the toilet.Little pieces of the obstruction should fly out from the force of the water.
3) Clean out the rim jets next.Just follow the same procedure that you did with the siphon jet.
If this doesn’t work there are some more serious problems that need to be addressed such as a foreign object stuck in the bowl,closet bend or water closet branch or a venting problem.
I hope that this is of help to you!!
PATRICK J FOLLO