Weird fix for new toilet not flushing completely.
We're in the middle of several interlocking projects here, with one of the simpler ones being a toilet bowl replacement. After about three weeks of flawless operation it just refused to flush completely. The water would slowly change out, but even a piece of tissue paper was hard to get to got down.
This morning I snaked the toilet, no improvement. So I shrugged and pulled the whole thing out and set it up on a milk crate in my back yard. I could fill the tank from a hose and watch it flush... and it still didn't not flush completely. I blew out the drain with the hose, even snaked with a 2" piece of foam, no obstructions but no good flush either.
Pulling the tank and looking inside the water feed hole in the seat I see... some fitting for the tank supply tube I never needed that must have dropped inside during shipping. (Hey, I didn't put it there.) It was blocking the strong flow enough to prevent a siphon flush, only enough water came through for a slow change. I only saw this because I put a mirror down the hole on the seat, and I was only looking as I was completely out of any useful ideas what to try next.
Just something else to keep in mind for those weird toilet problems.