iphone will not scroll down

I've noticed in certain apps, that if the app is displaying more content than can fit on the screen, you cannot scroll down to view it. This is mainly a problem when you need to click something and can't get to it. The only remedy is to switch to my desktop computer. I have tried various settings and it seems to be poorly written code by the developers of the apps. I do have a model with a slightly smaller screen than the newer devices, but that should be taken care of automatically shouldn't it? So that's why I am puzzled. Anyway, aren't the majority of apps really just using mobile Safari under the covers?

What I've tried:

1) font / text size

2) Display Zoom (I use Zoomed, but I've changed it to Standard before)

3) accessibility settings

a) tap the back of the screen to scroll down, then QUICKLY tap the button I need, because it scrolls right back up FAST. I got this to work once on one app.

b) zoom / magnify

c) larger text "OFF" (I use "ON")

I'm open to other suggestions. For now it's inconvenient and annoying.