Perhaps the powder bridged in the measure. Do you give it a good tap top and bottom when you cycle the measure? The vibration helps the powder drop more consistently. Some powders are worse than others about this. What kind of powder were you using?
But if it did bridge in the measure then other rounds would be likely to be heavier charge than intended.
"Always place your clothes and weapons where you can find them in the dark"
"Understand that the enemy is not the enemy in his own eyes ;this may offer you an opportunity to make him your friend. If not, you can kill him without hate, and quickly."