Moisture in the air can swell dowel pins so they don't fit. Forcing them runs the risk of splitting the bored pieces.
joinery shop tip

Shrink those dowels by placing them on a paper towel inside a microwave oven set on HIGH for 30 seconds. Remove the dowels, and check them for fit. Microwave them for an additional 15 to 30 seconds if necessary.
—From the WOOD magazine shop