Why aren't Flash or Java detected when I know that I have installed them?
"I know that I have installed the Flash and/or Java plugins, but the detection pages (Java detection, Flash detection) it say that they're not installed. Why is this?"
Why isn't Flash detected?
The most common reason for this situation is that you're running Google Chrome - this is because, by default Chrome blocks Flash from running (or being detected).
Why is Flash blocked by default?
Flash has a terrible security track-record, and vulnerabilities continue to be discovered.
Fortunately Adobe (the company that makes the Flash player) continues to fix (known as "patching") these vulnerabilities as they are discovered, however there is still always a lag-time between the security problem being discovered and the patch being made available - during which, anyone with Flash would be vulnerable to the security exploit. And that's still hoping that everyone updates their copy of Flash instantly as well - which often doesn't happen!
On top of this - not all security vulnerabilities are disclosed by those who find them; sometimes they are kept secret and used by criminals.
Combined with the fact that Flash is continuing to decline in popularity - due largely to the rise in mobile devices which don't support it and the emergence of new technology which is able to replace it, Google have decided to disable Flash in their Chrome web browser, starting with Chrome 56.
Adobe themselves released a statement in November 2015 encouraging everyone to stop using Flash for modern web applications, in favour of newer technologies like HTML5.
As such, except for a few big-name websites (including Youtube and Facebook), Chrome will now not automatically run Flash files unless you specifically allow it to. As such, unless you whitelisted whatismybrowser.com, we too won't be able to detect the presence or version number of your copy of Flash Player.
Why isn't Java detected?
Java can be installed in more than one way on your computer. Java can exist as a stand-alone program, and it can also exist as a Java Plugin integrated with your web browser.
When it was developed in the mid 1990's, it was a cool way of adding advanced interactivity to web pages (by 90's standards!) I still remember making web pages with "cool" navigation effects and experimenting with making (awful) browser based games! It was amazing at the time, but now, the NPAPI interface is an old and out-dated technology, and some web browsers such as Chrome have stopped supporting it.
Starting with Chrome 42 (released April 2015), Chrome is not able to run any NPAPI-based plugins - most noticably the Java plugin.
However this is widely regarded as a good thing. Similar to the Flash plugin, Java has a terrible security track record, and also similar to Flash, it continues to be abandoned in favour of newer technologies (in fact, it was Flash in the 2000's which originally supplanted Java's dominance as a browser plugin!)
So even though you might have downloaded and installed the Java Runtime Environment, if you're using Chrome or some other modern browsers, you will find that your web browser can't run Java files, nor will Java report as being detected.