What causes soot to flow inside lance tube?

i have got a problem with long retractable type soot blowers. the soot blower lance tubes contain alot of soot though the design pressure of sealing air that is 600 mm of h2o is maintained the furnace pressure is 300 mm of h20. the scavenging air is also provided from the same header. inside the seal box there is a annular chamber(formed due to the presence of lance tube) around which seal air distributes and pass through six seal plates to atmosphere and one seal plate to furnace . the nozzle of the lance stays inside that chamber with its nozzles on the same chamber.WHAT CAUSES THE SOOT TO FLOW INSIDE THE LANCE TUBE?