No air through heater core?

Hi all, i have a 97 buick Century and when i turn the driver or passenger climate control knobs to heat, the air stops flowing. This is what i've done so far: removed the heater core cover, verified motors controlling both driver and passenger valves are working(i could hear the motors and feel the valves moving), replaced the cabin air filter. Looking at the diagrams in the repair manual(WPB-3), i believe there is a blockage at the heater core somehow. What could this blockage be? My next step will be to remove the heater core and clean the coils. Is their any other areas i should be looking at before removing the core? Thanks in advance for any assistance.
Ray

Just in case anybody is having a similar problem, i've finally figured out the problem. Something i didn't notice before was that the temperature needle was fluctuating between the middle of the gauge and the hot side. This was an indication to me that the thermostat was not operating properly. So, i replaced the thermostat and what do you know, everything is now working properly.