Zooming in Safari not working on all sites

Hi

I use my iPhone 6 Plus all the time. I like the large screen as it makes things easy to read. In Safari I enjoy the reader view function as that make pages very very user friendly. For pages that don´t have that feature I use stretch to zoom., but on some sites this feature does not work. Is there a work around, or is there an app I can download in order to zoom these sites?

Thanks in advance.