One simple test for determining where the problem is, is to take a mop bucket, fill it with water and pour it into the bowl. If the toilet flushes properly, its not a venting problem but an adjustment or flapper problem.
The problem with new toilets is they have pretty specific flow requirements. If the level in the tank is one inch too low, it will not flush. If the flapper does not stay up long enough, it will not flush. If the water does not leave the tank fast enough, it will not flush. If the fill valve does not refill the bowl to the right height after the flush, it will not flush.
The water level should be 1/2 inch below the top of the overflow tube in the center of the toilet. The flapper should release all of the contents of the tank. There should be a tube from the fill valve (ballcock) sticking into the overflow tube, pull it up and make sure a steady strong stream of water is pouring into the tube.
If the toilet doesnt flush properly, when you pour the bucket of water into the toilet, there is probably a venting problem.