My touch and basic function is a bit unresponsive sometimes

so i know its 2020 and kind of late for an iphone 6s problem but here i am, since 6s is even updatable to ios 14 so its still a good phone i bought it for my mom for her birthday and yes i bought it as a used device, i tried my best to check it and its battery is also replaced. After a few days, my mom complained that the apps are unresponsive which i looked up and they were working fine then again after a few days (the seller gave me 7 days of warranty) after around 10 days, I tried to use the phone and its updated to ios 13.6 now, the bottom part of apps mainly like facebook are unresponsive sometimes, the touch on homescreen works flawlessly, and on sometimes the small back button on top left corner is also unresponsive (i have a glass protector on it) and sometimes on the homescreen, the control center does not opens by swiping, the touch works on the bottom but the control center just doesnt and the most interesting part is that right when i lock and unlok the phone, everything seems to work as good as new. Then the unresponsive touch like this appears agains after a few days and a lock and unlock fixes it. Its kind of annoying thats why asking what could be the fix? I really dont wanna keep the protector off cuz my mom’s usage is rough and i bought that phone from my 2 years worth of savings so dont want to see it break so soon. (I bought the phone on 10th june 2020)