If you need the fracture chunks to break apart further into smaller pieces you’ll want to use PhysX Labs (standalone app or 3Ds /Maya Plugin) to get more than one depth layer for destruction along with the ability to use custom masks for specific looks for your DMs.
Try setting the debris life to be a little longer. Also, you can try using the “Fracture Impulse Scale.” You may need a high value to start with and then work your way down to a lower value to fine tune. (By high value, I would suggest starting at a few thousand or so, if you don’t see results try higher until you see results)
Do you have yours DMs set to simulate physics and to Start Awake? Try unchecking Start Awake to fix this. This may work. Hard to say without knowing your specific settings.
If you want the player to deal with the rubble, then using the debris timeout settings above would probably not be a good thing unless you set them to a higher value that is after the player would have effectively dealt with them. In my experience using Debris timeout is better for objects that the player won’t need to mess with.
It’s a good thing to remember that even though you fracture something and it’s now a DM that the game doesn’t assume these things. If something is not behaving properly that means that it’s been told somewhere via a value or checkbox that it needs to use this behavior. Narrowing down what settings work and what don’t is part of tweaking to get the final results needed.