do you have any vents on this new bathroom you installed? was it inspected? did a plumber complete the work? when you flush the toilet, does the water come all the way to the rim? swirl? is the closet bend holding water or the drain? did the toilet come from home depot? if the drain is done properly, then the toilet is defective. There isnt a test plug anywhere in the line is there? the line could be partially stopped up, taking water from a bucket but not from a toilet flushing.