About the only way this can happen is that you had the racking arm (tube) pointing UP when you transferred wort into the fermenter (our instructions clearly indicate the proper orientation but you might have missed that). Then you get some trub that will settle down into the tube, solidify, and thus clog the tube. And/or you are in the process of racking beer off from the Brew Bucket and you get down to the dregs at very end there and you are pulling up bunch of trub anyway - in which case a clog is good as you don't want that stuff in your final product ;)
So if you find yourself with a Bucket full of beer you want to rack out of there, and you open the valve and only a trickle (or nothing) comes out, then this is what you do:
- Get yourself a piece of wire (coat hanger, copper wire, etc) and hit it with a propane torch and/or sanitize however you feel best.
- Open the valve (nothing is coming out at this point, still clogged)
- Poke the wire up into the valve with the idea of having it reach all the way up into the racking arm (tube) to dislodge the blockage
- You'll get some beer flowing out by this point, so quickly pull out your wire
- Then turn off the spigot. At this point you should be unclogged and ready to continue with the racking process
This won't ever happen if you transfer wort in with the racking arm in the right orientation (down, not up where stuff can get into it)