Toilet not flushing properly

I have a new Kohler toilet that seems to have stopped flushing properly. When i flush it, instead of the water going down the pipe it just fills the bowl, then slowly empties out over about 20 mins, but obviously not flushing the waste away. Considering it was working fine yesterday, am somewhat mystified. Any tips on how to sort this out?

It could be something in the trapway.

Using a plunger

Or pouring a five gallon bucket of water in the bowl may get things moving again.