Make sure the collection system blower on the mower deck is turning the correct direction. Often, the blower belt hooks to the outer mower spindle pulley which is "double stacked" by being mounted directly on top of the deck pulley. Usually, the belt is crossed before being hooked to the top pulley, to create the proper belt tension and also spin the blower in the correct direction.
Double check the belt positioning as having the blower belt installed incorrectly will still cause the blower to spin, it will just turn the blower backwards, will also cause leaves and debris to come out under the area of the deck where the blower is attached. It's easy to do, as I have done that myself a number of times.
There a couple of different types of baggers, including a type that has a blower that mounts over the discharge chute of the deck. That's the type Sully Bear is talking about when he mentions the belt and double pulley.
Maybe yours is the type without a blower and just uses the airflow from the mower blades to blow the leaves and grass through a tube into the bag. If so there's no need to research the belt and pulley issues.