The most likely culprit is that there's a blockage in the flange, but it could also be that the water level is set too low in the tank because your landlord is a cheap bastard. Check inside the tank, if the water level is below where the line is (there's usually a water line stain in the tank) then the issue is water level, if not it's a blocked flange or drain. If it's the former, the flushing hardware will need replaced, the latter and all you need is a plunger.
If it's the water level there might be a simple fix by removing a brick or something in the tank that was put there to reduce the water amount that is in the tank, or by unbending a metal rod that holds the float.
The most likely culprit is that there's a blockage in the flange, but it could also be that the water level is set too low in the tank because your landlord is a cheap bastard. Check inside the tank, if the water level is below where the line is (there's usually a water line stain in the tank) then the issue is water level, if not it's a blocked flange or drain. If it's the former, the flushing hardware will need replaced, the latter and all you need is a plunger.
If it's the water level there might be a simple fix by removing a brick or something in the tank that was put there to reduce the water amount that is in the tank, or by unbending a metal rod that holds the float.
At least some are adjustable with a screw that changes the length of the rod holding the float, I think.
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https://youtu.be/Pvj9QJDy1MQ It's probably one of the simplest home repairs you can do.