I have cold air, but I have weak air flow at the vents. What could be the problem?

Blocked cold air return.
Ice build up on indoor A/C coil. (can happen when all the grills/discharge air vents are not open).
Blower motor running on wrong speed.
Dirty air filter.

Maybe a dirty coil,,

Blocked cold air return.
Ice build up on indoor A/C coil. (can happen when all the grills/discharge air vents are not open).
Blower motor running on wrong speed.
Dirty air filter.

A low refrigerant charge could also be causing ice buildup on the indoor coil dropping the air flow.

Start with a visual inspection of your supply ducts. If one is broke and came apart you won't have any static pressure build up, if all looks good with the supply duct the first,second and third answers on the page is worth checking.