Exactly which browser are you using?
If Windows and either Edge or Internet Explorer, Microsoft does not allow external installs of Flash, the process is locked and managed by Microsoft
All major browser vendors block Flash by default, forcing users to enable it, usually on a per-site basis. Some browsers (e.g. Chrome, Edge) force you to allow it after each session (e.g. Chrome deletes your 'allow' setting when the browser is closed).
You don't provide any OS, browser information so we don't know what you're using. It sounds like the site you're attempting to view requires a login to access, so we wouldn't be able to attempt to reproduce the issue.
It's also quite possible the content developer hasn't kept up with the constant changes browser vendor's are making to Flash detection, which then prompts users to enable/allow Flash. You may want to contact the content developer regarding your issue.
The Flash Player Help tech note contains links to instructions on enabling Flash in the various browsers. Again, some browsers delete your Flash setting when the browser is closed, forcing you to enable Flash again on a subsequent session.
OK, I'm using chrome, i have OS windows 10. The problem is, that my computer shows my, that i've already installed adobe flash, that's ok. I also allowed flash on the site everytime after i had installed flash. The problem is, that even when i installed flash i allowed it, it still shows me this
I visited the same page yesterday and worked fine, no problem with it. Yesterday i didnt even need to allow flash on the site and it worked. But today, there is a problem even after multiple reinstallations. Here is a link to the webside https://radio.lf1.cuni.cz/prope . You don't need any acces, just click for example on this link Přednáška 07 - Zobrazování hlavy a krku and it should show you presentation.
Ok there is a way different problem, because now it works and i really don't know why. Like 30 minutes ago, it didn't work and now yes.