Just noticed the other day when using the stylus that the middle of the bottom half of the screen is unresponsive, if i use screen write and scribble lined horizontaly and verticaly there is about a centimetre area where no lines apear, get real anoyinh when your writing emails or texts as this is the area most used. just over 3 months old.
That's not good.
Assuming a soft reboot by turning it off and on does not help and the phone has not been dropped and the water sensors have not been tripped I would suggest visiting a Samsung Service Centre or Samsung Experience Store where they'll check it over under the 24 Month Manufacturing Warranty cover.
Current Phone ~ S8 Model: SM-G950F. CsC:BTU.
Samsung Gear s3 Frontier Watch
As well as what @BandOfBrothers has said you can also try a quick hardware test on your phone:
If the screen is still unresponsive in parts then you can find your nearest Support Centre, HERE.