I have a stuck sparge... What happened???

Lots of things can play into a stuck sparge:

1. If 5% or more of the mash is flaked or rolled barley, wheat, corn, oats, rye, or there is a lot of sticky malt types such as Spelt malt, then rice hulls MUST be used.
2. If the grain crush is too fine, or the mash temperature is too high - it can happen with ANY malt
3. Doing too much vorlauf, which over-compacts the grain bed
4. Using a pump (especially in RIMS setup) which can REALLY compact the grain bed.
5. Sparge water that is hotter than 170 degrees

This is a link to a pretty good article on the subject:

http://byo.com/hops/item/1529-troubleshooting-a-stuck-sparge

It is 18 years old, but still holds true today!