Please do not run things downloaded from the internet as admin. A split second window opening can be a sign of malware.
What do you mean with installed? With the Itch app?
And what do you mean with most games? Web games?
Games on Itch have in common, that they were uploaded on Itch. There, the common features end. So "most games" tells nothing.
Are you using the Sandbox feature of the Itch app and did not configure it yet?
Did you try starting the games without the Itch app?
Did you look in the game directory and look for an error log?
If some games run, and others not, find the common thing they share. Like being a unity game or whatever.