Weird toilet flushing problem

I have a two-story house which is about 15 years old. Two toilets upstairs and two toilets downstairs. All with the same model toilet bowl and tank.

One of the downstairs toilets doesn't flush very well. But the symptom puzzles me.

When I flush the toilet, the water goes down a little bit slower comparing with other toilets in the house. Thus it didn't create "suction" strong enough.

I thought it might be clogged. I used a plunger to plunge it. After plunging, it flushed pretty well (as well as other toilets). Then I left the toilet unused for an hour and flush it again (just for testing purpose, there's no paper or waste in the toilet bowl). It seemed that the flushing back to "not-so-good" state.

Maybe it's just red herring. But what is the best way to isolate the problem?

Thanks!

The water inlet holes around the rim may be clogged.

A plunger did not remove the blockage.

havasu said:

A plunger did not remove the blockage.

Click to expand...

I understand that. But why the toilet flush well right after plunging? Thanks!

There's something lodged in the ballcock of the toilet when you plunged it moved it out of the way and after the water stopped it came to rest back in same spot see if you can use alot of paper and if it gets clogged there's something in toilet, like a toy or marker or phone or whatever

Kaufman790 said:

There's something lodged in the ballcock of the toilet when you plunged it moved it out of the way and after the water stopped it came to rest back in same spot see if you can use alot of paper and if it gets clogged there's something in toilet, like a toy or marker or phone or whatever

Click to expand...

A ballcock is another name for the fill valve which is in the toilet tank not the bowl.

In the trap of the bowl not ballcock my mistake