When your furnace is turned on but there are heating air flow problems, there is definitely a mechanical problem. The culprit could be a worn out fan belt, or a bearing that is failing.
Another possible reason for the heating air flow problems could be there is an issue with the fan motor, such as a worn out part. Again, all of these possibilities are mechanical in nature and can be complex.
So we recommend bringing in a licensed Broomfield, CO heating and a/c expert.