It could be any number of things ranging from parts in the tank that are not set or, working properly, clogged internal passages from the tank to the bowl, to a clogged toilet.
Only going through some troubleshooting will you find the problem. More information is needed for us to say anything.
Using a 5 gallon pail about 3/4 full and pouring the water directly into the bowl as fast as possible will reveal if the toilet is clogged or, if there is a tank to bowl problem. Try it and report back.