Is your false bottom the right size? It should cover just about all of the bottom of the MLT, leaving just a very narrow band (about 1/4 ") around the perimeter. If it is too small, it will be easy to get grain underneath.
Is it plastic? If so, it is quite easy to get grain underneath. The following procedure helps to overcome this:
1. Use a good stiff tube to connect the false bottom to the MLT outlet. I use copper pipe connected at either end by a short length of braided tube. This helps to keep the bottom in position.
2. Before adding any grain to the MLT add a sufficient foundation of strike water so that the outlet tube is completely submerged. Then draw off a small amount of water to flood the outlet. This prevents air in the outlet from trying to float the bottom.
3. Don't stir too vigorously. If you catch the outlet, you can lift the bottom, and get grain underneath. Stirring a bit more gently for a bit longer works just as well.
4. Alternate between adding small amounts of strike water and malt, making sure that the malt is always suspended by the water, ensuring that the grain is thoroughly wetted after each addition before adding any more grain. This prevents the weight of unsupported grain from plugging the holes in the bottom.
Finally, if you do get a stuck sparge, you can always ladle the entire mash into your kettle or a bucket, clean out the obstruction, reassemble, and ladle it back again. It's a PITA, but still better than dumping a brew.
-a.