Put some food color into the tank and wait about 10-30 min. then check your bowl. If there's color traces in the bowl then the flapper is usually to blame. I suspect that is your problem. Usually replacing the flapper will solve the problem, but it is usually only on older toilets. I guess it could be an issue on a new toilet. The flapper is about $3.50 at most stores so it's a cheap, easy part to replace.