weak flush on kohler toilet?

I have a Kohler toilet (1.6 gal). It used to flush fine. We had our water shut off at 1 point around the same time we had a big clog. I plunged it and seemed like the clog was removed. BUt since then the bowl will not fill up as much as it used although the tank still does.

When I flush it is very weak, there is often a kickback of air and water as if it is still backed up. The fill valve seems to be working properly.

Any thoughts or suggestions would be appreciated. --

Andy
March 2012