Clean the siphon valve!
I'm bringing this up, because for me, it was nearly a miracle cure. Maybe it will help someone else. Here's the long story.
My toilet just would not fully empty. I made every adjustment, ran every test, cleaned out the jets with a coat hanger, etc. No improvement - there just wasnt enough water getting in there. When it got to the point that even multiple flushes wouldnt help, I was ready to just replace it. But this toilet is 22 months old. So what the heck?
Having nothing to lose, I went to YouTube to look for ideas before buying the replacement. I stumbled on to a vid that explained that there is a "siphon valve" at the bottom of the bowl in the front. Apparently this thing is crucial to getting enough flushing water into the bowl.
The vid said to put on a glove, stick your finger in the hole and scrape out the scale - various chemicals that condense out of tapwater as solids. Sure enough, there was literally a pile of this scale stuff filling much of that hole. This surprised me, because I dont have many hardwater problems with sinks, shower, etc. But anyway, I dug out that scale and then flushed. That toilet worked like new! Today I soaked the bowl in "heavy duty" vinegar (9% acetic acid) for a couple hours. The porcelain inside that hole is smooth and clean again. I'll be doing this with both toilets occasionally now.
There are several vids that explain how to do this if you want more info.
I hope this helps some other frustrated person.
Fred