I'm with Stevie on this one [mostly], although I don't know if I'd use the word "clog" to describe a slow draining mash. If it doesn't drain at all you probably have a clog at your filter/braid/false bottom. If it is draining, but slowly I suspect that it's some form of capillary action that retains the liquid we are trying to retrieve. If I leave my bag of spent grain suspended above a vessel without squeezing it wort will continue to drain out of the bag for several hours, how many I can't say for certain cuz I never left it until it finally quit draining. Hence, I squeeze the bag and move on to the next step. My limited experience is that rice hulls, even in small amounts, do hasten the rate at which wort drains from the mash, regardless of grist composition. Only once have I had a completely stuck mash, and that one was the result of such a dunce idea that I'm keeping the details to myself.