I'm assuming the filter is new.
Check the blower fan inside the unit in the house and make sure the blades aren't caked with dust. They also can wear down over time, so the motor itself could just be weak.
If you can access it, check the A-coil inside of the unit inside the house (easy way to find if it's not obvious is to follow the big and little copper lines into the heat exchanger and viola). If the coils are somewhat dirty, that can be the issue.
Make sure your ducting doesn't have any little flaps in it leading up to said vents that haven't closed over time (usually located in the basement.) They look like a small metal tab on the side of the ducting.
I'm basing this off of the average things I have dealt with, but I hope it helps.