When you get on the app to try and reserve a time, it says no availability but asks you to check back at 11am, 2pm and 4pm. But one tip is that another round opens up at 6pm as well, even though the app doesn't list it. I was able to grab a time for my family of 5 while sitting in my room at Cabana Bay, literally watching the clock and refreshing my phone waiting for the 6pm slots to open. Another tip is that it gives you 2 return times, pick the second (later) one, because everyone will try and pick the first available slot by nature and if you choose that one you are more than likely going to get blocked out, because the times literally go away in seconds.
If I had not kept trying and gotten lucky, our family would have been locked out of being able to ride Hagrids this whole entire trip. Like many out of state tourists, we get to come to Universal once every 3 to 5 years. Being blocked out of a park's best ride because your fingers aren't fast enough on a phone is something that should never happen at any park. Its why I won't go to Disney Hollywood Studios. I say just do standby lines and if the wait is 3 hours, so be it. At least you can still get in line if you want and if you decide not to ride, then its your own choice.
One other thing of note, when you get a virtual line pass, it doesn't mean your wait will be any shorter. It just means you get the privilege of being able to wait another 2 hours to actually ride. Its important to know this ahead of time, as we had one family in front of us literally walk out of the line after after an hour because they were upset because they thought virtual queue meant it was like a fast pass line, that you would get to ride soon. I felt bad for them because I knew they were giving up too early and probably wouldn't get another chance to ride later.