I have a 4 zones each zone has there own units. The bedrooms ducts are blowing fine and strong (I changed the air filters ever 30 days ) its just that the bathroom has no airflow. I closed 2 ducts in 2 different bedrooms to see if that helps.. and I checked bathroom and there was a drop of more airflow but not as much as the air flow in the bedrooms... does anyone think this is a problem since they built it ? Or it can be fixed still?

Any thing can be fixed. Since this is a on site type of problem that need a visual inspection call a local professional and I am sure they can figure something out for you.

Chances are that you have a damper closed, or the ductwork is disconnected to the problem vent. If it's flex duct, mice and squirrels *love* to chew it apart and use the materiels for nesting. Check at the heating/cooling system that there are no closed dampers.

Since it's zoned (what type of zoning? VVT, or simple duct dampers), look at the actuating mechanisms. Is the bathroom on its own zone? If it is, are the damper actuators working? Have the dampers "skipped" or moved out of position?

If it's not on its own zone, chances are that the ductwork has separated and the air you seek is in your bathroom wall space or ceiling space. Is the ceiling space around the vent warm to the touch, but the walls are cool? Check wall and ceiling temps with an infrared thermometer if you have one available. If not, use a clean dry hand and feel the walls and ceiling. If the ceiling is much warmer than the walls-the ductwork has separated and your warm air is in the ceiling!

As HVAC1000 previously stated, this will need onsite inspection, or you could possibly cost yourself twice the bucks and still not have it fixed.

I hope this helps.