Apple doesn’t publicize this one much… they must be embarrassed, is it affects a large # of users and apparently Apple does not want to spend the money to fix this problem.
Most of today’s phones (2019) use capacitive touch screens. These screen do not work by pressure, they detect electrical charge changes.
If your fingers are very dry – iPhones have a hard time recognizing a touch, as an change is the electrical field is not completed with very dry hands. I’ve heard this referred to as “zombie fingers”… but truth is lots of people have dry fingers at different times in their lives (especially older people, or people that wash there hands quite often).
I do wash my hands frequently and I am an older adult, and found my new iPhone was not recognizing touches, taps, double taps etc.
Hand lotion works wonders if that is available. A compatible stylus will work too… more on that to come. Push comes to shove, lick your finger… as a last resort, that will help for a tap or two.