It is not unusual for UAC notifications to popup and remain invisible! You hear the system sound, if you have one configured, but you are not able to see the dialogue and select an option, to run a program or cancel its execution.

I have not figured out a way to make Windows bring up always visible those UAC notifications. However, there is a way to do your job, which after all is the most important.

When you run a program and you are unable to see the UAC dialogue, just press Alt+Tab to switch to another open application, and immediately press again Alt+Tab to return to the application you tried to run. Now the UAC dialogue is visible and you can select the option you prefer!

Enjoy!