Countrypistol is on the right track. All wake ups with a down command first have a problem- they can conflict with the delayed wake-up command (which is down). So far as getting the command grab on wake-up, it's about timing. There are two windows on knockdown. The first is delayed wake-up/tech roll. The second that comes slightly later is when you can wake-up attack. (You can do this because the delayed wake-up window is now closed).
The best way to find this is to practice. The animations are your guide, here. Just set the AI to do different types of knockdowns and learn the animations for your windows. I like to watch my character's feet. The instant they go still, that's usually when you can wake-up attack in the second window. It probably varies across characters, though. I recommend using the animations because if you play online, well, the lag will change the timing but the game still relies on the animations. I just find it to be a more reliable option.
Good luck!