No reinforcements were used by Union forces, thinking they weren't needed.
The North had thought, going into the Civil War, that victory would be relatively easy to achieve. According to the History Channel, the outcome of the battle (a victory for the South) "sent northerners--who had expected a quick, decisive victory--reeling."
The First Battle of Bull Run is called the First Battle of Manassas by Southerners, after the city near where it was fought. The battle took place in Virginia in July of 1861. The Confederate forces received reinforcements during the battle and that helped them in achieving the victory.