low Air flow would most likely be one of the following. (in order of probability in this case)
1. Fan needs oil. (even sealed units need oil eventually)
2. Fan speed controller bad. (but that will most likely not make any air)
3. Air filter. (but you probably changed it)
4. Icing. (but that takes time. )
Things to try.
1. Turn on the furnace and see if the fan turns on.
If it does and blows good then it's #3 or #4
PS: After this air flow problem is fixed... Don't forget to clean the outdoor condenser unit as a clean condenser is more efficient and uses less electricity.
You can have it professionally cleaned or just use a garden hose to clean it.