Partial blockage?

Hi,
I have a recently installed (2 weeks) TOTO Carrollton that, since installation, has been flushing incompletely and getting progressively worse. Nothing has been dropped into toilet and a large volume of water poured into the bowl goes down relatively easy, though not completely. Called plumber who installed toilet--he removed toilet, replaced gasket, snaked, etc. Flushing is significantly worse now. He is scratching his head, thinks that I should buy a new toilet. This seems to be a reliable toilet, is this plumbing issue really rocket science?
Tessa

Considering it's a TOTO, I'd be doubtful that the toilet is to blame. Do other drains in the house drain slow, like a bathtub or sink in the same room? Either something's caught in the toilet or further down in the drain line. There's a 0.0000001% chance that it's the toilet, but I'd look elsewhere first.

Did he auger the toilet and / or snake the line leading away from the toilet? It almost sounds like a vent problem.