As far as I know the 1.6 gal flush toilets are code everywhere in this country. While most of the newer toilets don’t have a problem flushing, some do. The first thing to do is to make sure the water level in your tank is is set to the correct level. If that doesn’t work, you might replace the toilet with a new one, but make sure the inner surface of the integral trap is fully glazed.
If all else fails, I’ve heard of people going to Canada. . . . .