it is impossible to diagnose.
In addition, 'intermittent' problems make the task that much more difficult.
What we would need to know;
1] Which browser? If this happens with one browser, does it happen with others?
2] Which browser settings? For example, does the browser normally download and run automatically, or does it give you the option to either "Download and run" or "Download and save"?
3] If you always use the "Download and save" option, which location are downloaded files saved to?
4] What does, "Unable to install" mean? Any error messages?
5] Which files? Any examples of which files fail to install, and where you downloaded from, (eg url web site addresses)?
6] Do you ave any security software installed that may be blocking downloads and/or installs?
I have more than once been unable to install the program I wanted for a reason that I do not know.
Other programs readily install.
Is their something I can do about this.