This really shouldn't happen since I tested the recipe specifically to avoid this. To prevent this from happening, PLEASE do not over pulse your ingredients. I did this the first time and it turned to mush and wouldn't firm up. Leaving chunks somehow helps it to come together. Also, ensure that your toothpick comes out pretty much clean to confirm everything is cooking together. Be sure to properly press this into the pan too. Ensure you're using the right size pan since a too wide meatloaf will not hold up properly. Let it sit for at least 10 minutes to cool at room temperature. If you've done all of this and it's still giving problems, then last resort, if you sit your meatloaf in the fridge for like an hour, it will firm up completely. Then slice it up. I cut my slices about 1-inch thick.