Toilet "gives back" what was flushed several hours before (Kohler 2-pc)

[KarenTheBold]

Kohler 2-pc toilet was installed 20 years ago in a MIL unit over my garage and worked fine.
Now I'm using that area as a home office, so using the toilet from time to time.

I will use and flush the toilet, and the next day--there is stuff in the toilet bowl. It's almost like I haven't flushed, but I did (visual verification, too). The innards have been replaced a few times over the years, so the insides are fine and the flush 1.6 g is normal.

Question: Does this mean there is a partial blockage in drain or the sewer line? Or what? (I want to fix the right problem.)

 

[Terry]

Sometimes things get left in the trapway which can slow the flush down.
Also, the small port at the bottom of the bowl for the siphon jet can sometimes get blocked preventing that jet of water to start a good siphon.
A rubber glove is a good thing to have when putting your finger there to clear it, or at least check it. I have seen that a few times on the modern siphon jet bowls.