The query was not very specific on the location of the blockage, and the type of blockage (physical compaction, chemical reaction, or condensation agglomeration) was not spelled out.  However, let me offer some direction:
 

  1. If the blockage is in the inlet or around the upper cylinder and made up of physically compacted solids, the velocity may be too high.  FCC units usually have multiple cyclones in series to avoid this problem and still get good particle collection.  Also, the solids may be building on the roof due to the stagnant areas and some of the reactions may be continuing.  I would look back at past operations and see if the velocity has changed.  If the blockage is due to condensed solids or reaction, there may be some unexpected heat losses or gains.
     
  2. If the blockage is near the solids outlet, there may be condensation of gases depending on the reaction (exothermic or endothermic) and how far the cyclone is from the disengagement zone of the fluid bed.  Also, they may have made a change in operations (throughput or volume of gas flow) that makes the gas velocity lower and this will make the inner vortex shorter than normal.  The result is the lack of shear on the surface that keeps the solids flowing and the solids flow in slugs or stay on the surface for long periods of time.  It is like having funnel flow rather than mass flow in a bin.  The potential for this to happen depends on the style of cyclone (long or short cone with a long or short barrel).
     
  3. The dip legs may be too short or too shallow in the fluid bed.  This may allow them to leak and allow gases to come back up into the cyclone.  The reaction that is supposed to take place in the fluid bed may be taking place in the cyclone.  Also, this may give a different selectivity to the reactants.

A step wise approach would be: First look at the velocity and composition changes that can influence the inlet velocity.  Second, It may be necessary to look at composition of the exhaust gases to identify any leakage.  Third, they would need to take a sample of the solids and look at them under a microscope to determine if the particles are agglomerated.  Depending on the composition of the catalyst, they could run a TEM (transmission electron microscope) analysis to see if the points of contact are the same composition or a condensate from the gases.  Finally, look for heat losses that may be changing the velocity and potential for condensation.