Kohler Veil on wall toilet

I am looking for some advice on a particular type of toilet. I recently remodeled my basement and decoded to redo the bathroom while we were at it. The bathroom in the basement was an afterthought in my 1977 split level. We decided to allow for a bit more room we would spend the money and put the toilet under the stairs via an on wall toilet system. We used the 2x4 wall carrier and the Kohler Veil toilet. My contractor has installed it per the instructions and tiled both the three feet of pipe from the original location and the wall the toilet rests on. Now when. Flushing this toilet it does not evacuate the bowl leaving solids behind and forcing you to have to flush the toilet twice to remove the pee. There is always some water in the bottom of the toilet and kohler seems to think it's the plumbing. We have gotten a replacement casting from kohler as my contractor installed a vent pipe right behind the toilet and the bucket of water test goes right down. Can someone watch this youtube video of it outside the wall and tell me if this even proves my case for a bad casting? https://youtu.be/0jubVh_uvfk Any help is appreciated. Thank you.

If it is any help, here is the INSTALL GUIDE- http://www.us.kohler.com/webassets/kpna/catalog/pdf/en/1145594_2.pdf

Scroll down to pages 14 & 15 for troubleshooting.

I would first be looking at if the tank is delivering enough water quickly enough, and then checking to make sure that the drainage line does not have a partial blockage. Then move on to a possible bad bowl casting.