Check the filter.

How did you make this determination. Did you use the total sqin of the return"s and measure the airflow? Or are you just guessing. Unless you hear a air noise the return is probably working as expected. If you hear a noise, the return is not large enough. If you have too much filter thyen the return may not be strong enough. If this is the case increase the air flow. Either speed up the motor, or use a larger motor. For me, (20+ years of experience) I would not recognize a underused return, if it jumped up and knocked me down. I would recognize an undersized return by the noise.

We changed the filter and that's when we noticed the problem. Normally, the filter sucks into the return but it didn't this time. We can barely even hear the ac come on because all the pressure is so low. This house was built in 2003 and this is the first time we've had a problem like this.

The blower wheel could be dirty. If the wheel veins get loaded with dirt they will not push the air. If the filter sucks into the return your duct system has been all screwed up from day one.

If it is a 90% furnace the secondary heat exchanger could be plugged with dirt.

Get someone to look at it.

The best way to trouble shoot airflow problems is by checking static pressures. Read the linked page & ask the right questions. udarrell _ 2007

Here is a link that might be useful: udarrell.com