Fan is not pushing enough air in bed rooms at first floor

You may have dampers in your duct system which could be adjusted to increase airflow or your register grilles could be adjusted by decreasing airflow to a room that has too much air and increasing the baffle in a room that does not.

Hi,
Thanks for asking a question here, and Welcome to the site.
Do you have access to the filter? Is this a central unit? Check your exhaust filter, it could be dirty.
Has this just started happening? Or, has it always been this way?
Please post back, with more details. Thank you.

You may also find adjustable dampers located in the ducts that allow air flow to be increased or decreased to certain branches. In the basement or close to the air handler, look closely at each duct, you may find small levers or handles. The damper blade is lined up with the handle, so if the handle is in line with the duct the damper is wide open, if it is perpendicular with the duct, then it is closed all the way.

Adjusting these dampers can allow more or less air flow in those ducts to certain areas.

You may have motorized dampers also, you may find an orange or black box mounted to the ducts with wires connected to it, these are not usually manually operated, are controlled by the control system and thermostat, and if not working correctly will need a tech to check, adjust, and repair.