What is wrong with my new(ish) central air unit?

4 or 5 years back I bought a new central heat & air unit. In my hallway, it sounded like a roaring tornado & the cool air could blow the house down. It was great! Then after a few years, it now sounds hollow &/or shallow & barely blows any air. It is still cold air, but doesn't keep my house cool due to not enough air pressure flowing up through the floor vents. Used to you could see the air blowing things like curtains all around. Now, you can barely feel any air when you put your hand or foot over the vent. I have it serviced yearly & change the filters regularly. The service man said it was a little dirty on the outside unit & was running about 10 degrees hotter than normal. He cleaned it & said it would be fine now. But, like always it's NOT. Since I bought this new, he services it yearly for free. I hired someone else to service it this time, hoping they could figure out the problem. But, didn't help any. What can I do to get my house cold again? Any suggestions would be highly appreciated. Thank you.