I can chim in on this since I had this problem as well. What cartridge are you using? They have a new hard shell one that after switching to I have had good flow all of the time. Also I use to run the reactor with the GFO and Carbon both tumbling but after talking to Louis and ER he advised me to let the GFO tumble and to pack the carbon between the sponges. Also with the new cartrige you remove the spong for the GFO side since it had a hard plastic top with slots in it to catch the GFO. Check it out on BRS.

http://www.bulkreefsupply.com/store/...s-reactor.html