It's in the online FAQ and in the manual but sometimes people just don't do it right. Usually if you clear it out once good the problem does NOT recur as long as you are following the directions. It's actually fairly simple and straightforward.
"If you are following the directions to adjust the denitrator correctly it should always be able to get a strong flow when the flow valve is wide open. It is important to open the flow valve completely and then immediately decrease the flow to the correct rate each time you adjust it (every 2-3 days.) Bacteria can build up in the narrow tubing and this will clear out the tubing and valve of any bacterial build up. It will also let you accurately gauge the flow since when you check the flow rate the end of the outflow tube may be partially empty giving the appearance that the flow rate is less than it actually is. If the outflow has completely stopped even when the flow control valve is wide open then the outflow tubing and flow valve needs to be cleaned. Simply unscrew the outflow tubing and flow control valve and rinse it in hot water and blow on it until it is cleared. If that still doesn't clear it you can take a wire coat hanger and push it down into the outflow a few inches. Just make sure to do this to the outflow only and not the inflow. Also be careful not to lose any of the little pieces when you unscrew the outflow tube. "