When car ac barely blowing air from the vent there is problem with the air passage. Usually if the car air conditioning fan is at maximum setting the air coming out the car ac vent is also at maximum air blowing, but when the car air conditioning vent barely blows out air there is a problem, and the main reason of this is a blower problem or air duct blockage problem.
Whenever your car air conditioning barely blows out air from air conditioning vents, the first culprit that will come into a mechanics mind is the blower because most often than not the cause of the problem is a failing blower or sometimes sliding blower wheel from the blower motor shaft, or sometimes the reason is that the blower is suffering from a faulty dashboard switch assembly, in this case the blower will not operate on maximum operation even if it was set on the highest setting.
Same as for the air duct blockage this will restrict the flow of air from coming out of the car air conditioning vent, possible reason for this is that there is a closed or nearly closed air doors that is stuck inside from one of your car air conditioning duct, thus preventing the air to comes out your car air conditioning vent, also it is possible that an accumulation of debris such as leaves from a fresh air intakes is preventing the flow of air, but the restriction of air can only be felt when your car air conditioning is set to "fresh".
To correct the problem, first determine the setting of your car air conditioning system where the problem was appeared to diagnose the problem, if the problem is encountered only when your car air conditioning is set to "fresh" then the cause is accumulation of debris on the intake port for fresh air that comes from the outside, just check if there is an accumulation of debris such as leaves that restrict the flow of air to comes out from your air conditioning vents and remove it, the intake port is located on the outside beneath the cowl top, this is near the windshield base, just open your car hood to gain full access on the intake port.
If your car air conditioning barely blows out air even when your car air conditioning is set to "recirculate" then you have a more serious problem, if this is the case you will need to bring your car to a reputable shop specializing in auto ac repair and have it check by an expert mechanic.
Prior to checking the mechanic must first uninstall a lot of parts before the mechanic can actually check your car air conditioning blower and your car air conditioning ducts, which mean that it will need some expertise and can cost you a lot.