Sometimes a foreign object like a pen, comb, action figure, etc. Small children are often the cause of this, but we adults can knock things without realizing it, too. These things can lodge in the trap and catch paper which will slow the drain. Even snaking and augering can miss them. When the paper is gone, the object doesn't slow the water down so the flush appears normal. You may be able to determine if that is the cause if you pull the toilet out and go from the underside up into the trap. This may be worth a try.