Buttons doesn't work in sleep mode(sometimes)

OK so i bought this used iphone 5s couple of days ago and found out that it works fine on active mode. like all button (volume, power and home button) works fine, when it goes to sleep mode sometimes all of its buttons stop responding, it doesn't happen that all time, but lets say 6 out of 10 times it happens..either i have to wait and keep trying for like 1-2 mintues then they start working or put it on charger and then it works.

first i thought it was a problem in home button. but now i think its something else because all buttons stop responding. any suggestion, help would be much appreciated ^_^

thanks in advance.