I have the Phil's False Bottom in a 5g cooler, and have never had a problem like yours.
I have had problems stirring the mash, and dislodging the rubber bung that holds the outlet tube in place, but that's not a problem with the false bottom.
I've also had some stuck sparges, but only if I allow the grain bed to get too compacted, and when using a large amount of flaked barley. Again not a fault of the false bottom.
These are the rules that I follow.
1. Always add foundation water before adding grain. (I make sure there is at least 4 inches of water above the bottom before adding any grain.
2. Having added the foundation water, and before adding the grain, bleed a little water out of the outlet to eliminate any air in the outlet. (The air can cause an air lock which could be what happened to you.)
3. If using a large amount of flaked cereal, don't let the sparge water drop too low when sparging, as this can compact the grain bed.
4. Don't screw the elbow all the way in. If you do, it can rest on the bottom of the cooler and not allow wort to drain. (This is another possible cause of your problem.)
5. If you do get a stuck sparge, you can usually start it off again by backfilling with water through the outlet hose to force out any grains or air that are stuck in the elbow.
I've had mine for over 10 years, and am very happy with it.
-a.