Some of the water saver commodes as set to have more water in the tank than is allowed through for the flush. This is not a flapper adjustment, it is a design choice the manufacturer made.
I have a Kohler with a tank like this. There is so much water in the tank that you can hold the flush lever down (so the flapper does not close) and get a complete double flush (flush, bowl refill, flush and bowl refill) from one tank. This is not recommended, but I found this out by doing some experimenting on ways to make this flush better because it does not give a good flush. If you have a stoppage and hold the handle down, you will overflow the commode, so definitely do NOT do that.
My conclusion: If you want to correct this condition, replace the commode with a different model (which may have a different flushing problem). This is in our hall bathroom, so I left it, least I get a worse one when replacing this one.
This is one of the seemly consistent problems with many water saver commodes. They just do not flush well.