Sometimes things just don’t want to go down, but there is nothing unusual down there, and the plunger just isn’t helping…so what do you do?
While the water is slowly seeping down the drain, boil about a litre (or quart) of water. Pour about 3 T. of dishwater detergent into the toilet, then follow up with the boiling water. Let sit a moment. If the water level is high enough, gently plunge without splashing yourself. If not, flush, then plunge.
Most of the time this will work amazingly well. If it doesn’t, and you are sure there are no unusual objects in the trap, repeat this process.