HVAC not working properly?

When we got the vehicle, everything worked fine. Over time I have noticed when we have the temperature control on the defrost it blows hot air and the fan is at full blast like it should be. The problem is when you switch it to the face or the face and feet it will stay at temperature either hot or cold, but the fan doesn't blow as hard, and I notice I can feel a little bit of air coming out of the defrost. I have tried looking at the actuators, but I just don't know which one to replace i.E. The one on the left by the gas pedal. The one on the right etc. Any help would be appreciated.