I have two AC untis, the one for upper stair fine with little nosie.The downstair one used works very well, now it with very little air flow (still cold), with no return air flow at all (air from room). The vent might blocker by a sofer before, the filter was chagne recently. Any clue/ seems the fan broke.

I doupt if the units are belt drive but it is possible. Look in the blower compartment and see if the motor is connected to the blower by a belt.

Have you blocked any of the return grills?
Have you turned off any of the registers?
Are you sure all filters are new?
The blower wheel could have a build up of dirt in the fin areas and that will definitely stop the blower from blowing as hard as it used to.

Make sure all airways are open.